From 57ed0a2a53cc1ed5ed61bec6d0cbbc139e1b4542 Mon Sep 17 00:00:00 2001
From: Tim Chevalier <tjc@igalia.com>
Date: Thu, 20 Jun 2024 16:02:50 -0700
Subject: [PATCH] ICU-22794 MF2: Move .json files for tests into top-level
 testdata/ directory

Modify ICU4C and ICU4J test readers to handle all tests

Add `ignoreJava` and `ignoreCpp` properties to tests where needed

Includes parser bug fixes:

ICU4J: require a complex-body after declarations

ICU4J: Correctly parse the complex body after an unsupported statement

ICU4J: Handle date params in tests and remove default params for tests

ICU4J: Handle decimal params in tests

ICU4J: Require whitespace before variable/literal in reserved annotation

ICU4J: Require whitespace between options

ICU4J: Require a variable-expression in an .input declaration

ICU4J: don't require space between last key and pattern in variant

ICU4J: don't require space between selectors

ICU4J: allow whitespace after '=' in option

ICU4J: parse escape sequences in quoted literals according to grammar

ICU4J: allow whitespace within markup after attributes list
---
 icu4c/source/config/dist.mk                   |   2 +
 icu4c/source/test/intltest/intltest.cpp       |  55 +++
 icu4c/source/test/intltest/intltest.h         |   2 +
 .../intltest/messageformat2test_read_json.cpp |  45 +-
 .../source/test/testdata/message2/README.txt  |  40 --
 .../testdata/message2/invalid-options.json    |  10 -
 .../testdata/message2/reserved-syntax.json    |  39 --
 .../testdata/message2/resolution-errors.json  |   8 -
 .../message2/spec/test-functions.json         | 306 -------------
 .../message2/syntax-errors-diagnostics.json   | 352 ---------------
 .../dev/test/message2/Mf2FeaturesTest.java    |   8 +-
 .../java/com/ibm/icu/message2/MFParser.java   | 129 ++++--
 .../ibm/icu/dev/test/message2/CoreTest.java   |  23 +-
 .../message2/CustomFormatterPersonTest.java   |   2 +-
 .../test/message2/DataModelErrorsTest.java    |  25 +-
 .../icu/dev/test/message2/FunctionsTest.java  |  17 +-
 .../dev/test/message2/IcuFunctionsTest.java   |   9 +-
 .../dev/test/message2/MessageFormat2Test.java |  24 +-
 .../dev/test/message2/SerializationTest.java  |   6 +-
 .../dev/test/message2/SyntaxErrorsTest.java   |  21 +-
 .../ibm/icu/dev/test/message2/TestUtils.java  |  60 ++-
 .../com/ibm/icu/dev/test/message2/Unit.java   |   8 +-
 .../dev/test/message2/data-model-errors.json  |  32 --
 .../dev/test/message2/icu-parser-tests.json   |  61 ---
 .../icu/dev/test/message2/syntax-errors.json  |  56 ---
 .../ibm/icu/dev/test/message2/test-core.json  | 212 ---------
 icu4j/pom.xml                                 |   1 +
 testdata/message2/README.txt                  |  88 ++++
 .../message2/alias-selector-annotations.json  |   0
 .../message2/duplicate-declarations.json      |   0
 .../message2}/icu-parser-tests.json           |   4 +-
 .../message2/icu-test-functions.json          |  71 +++-
 .../message2/icu-test-previous-release.json   | 141 ++----
 .../message2/icu-test-selectors.json          |   4 +-
 .../invalid-number-literals-diagnostics.json  |   0
 testdata/message2/invalid-options.json        |  34 ++
 .../message2/markup.json                      |   0
 .../message2/matches-whitespace.json          |  18 +-
 .../message2/more-data-model-errors.json      |   4 -
 .../message2/more-functions.json              |  14 +-
 testdata/message2/more-syntax-errors.json     |   5 +
 testdata/message2/reserved-syntax.json        |  40 ++
 testdata/message2/resolution-errors.json      |  14 +
 .../message2/runtime-errors.json              |  12 +-
 .../message2/spec/data-model-errors.json      |   0
 .../message2/spec/syntax-errors.json          |   0
 .../message2/spec/test-core.json              |  21 +-
 .../message2/spec}/test-functions.json        |  68 +--
 .../syntax-errors-diagnostics-multiline.json  |   0
 .../message2/syntax-errors-diagnostics.json   | 402 ++++++++++++++++++
 .../message2/syntax-errors-end-of-input.json  |   0
 .../message2/tricky-declarations.json         |   0
 .../message2/valid-tests.json                 |  70 ++-
 53 files changed, 1119 insertions(+), 1444 deletions(-)
 delete mode 100644 icu4c/source/test/testdata/message2/README.txt
 delete mode 100644 icu4c/source/test/testdata/message2/invalid-options.json
 delete mode 100644 icu4c/source/test/testdata/message2/reserved-syntax.json
 delete mode 100644 icu4c/source/test/testdata/message2/resolution-errors.json
 delete mode 100644 icu4c/source/test/testdata/message2/spec/test-functions.json
 delete mode 100644 icu4c/source/test/testdata/message2/syntax-errors-diagnostics.json
 delete mode 100644 icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/data-model-errors.json
 delete mode 100644 icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-parser-tests.json
 delete mode 100644 icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/syntax-errors.json
 delete mode 100644 icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/test-core.json
 create mode 100644 testdata/message2/README.txt
 rename {icu4c/source/test/testdata => testdata}/message2/alias-selector-annotations.json (100%)
 rename {icu4c/source/test/testdata => testdata}/message2/duplicate-declarations.json (100%)
 rename {icu4c/source/test/testdata/message2/icu4j => testdata/message2}/icu-parser-tests.json (98%)
 rename {icu4j/main/core/src/test/resources/com/ibm/icu/dev/test => testdata}/message2/icu-test-functions.json (66%)
 rename {icu4j/main/core/src/test/resources/com/ibm/icu/dev/test => testdata}/message2/icu-test-previous-release.json (70%)
 rename {icu4j/main/core/src/test/resources/com/ibm/icu/dev/test => testdata}/message2/icu-test-selectors.json (98%)
 rename {icu4c/source/test/testdata => testdata}/message2/invalid-number-literals-diagnostics.json (100%)
 create mode 100644 testdata/message2/invalid-options.json
 rename {icu4c/source/test/testdata => testdata}/message2/markup.json (100%)
 rename {icu4c/source/test/testdata => testdata}/message2/matches-whitespace.json (82%)
 rename {icu4c/source/test/testdata => testdata}/message2/more-data-model-errors.json (94%)
 rename {icu4c/source/test/testdata => testdata}/message2/more-functions.json (87%)
 create mode 100644 testdata/message2/more-syntax-errors.json
 create mode 100644 testdata/message2/reserved-syntax.json
 create mode 100644 testdata/message2/resolution-errors.json
 rename {icu4c/source/test/testdata => testdata}/message2/runtime-errors.json (61%)
 rename {icu4c/source/test/testdata => testdata}/message2/spec/data-model-errors.json (100%)
 rename {icu4c/source/test/testdata => testdata}/message2/spec/syntax-errors.json (100%)
 rename {icu4c/source/test/testdata => testdata}/message2/spec/test-core.json (83%)
 rename {icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2 => testdata/message2/spec}/test-functions.json (77%)
 rename {icu4c/source/test/testdata => testdata}/message2/syntax-errors-diagnostics-multiline.json (100%)
 create mode 100644 testdata/message2/syntax-errors-diagnostics.json
 rename {icu4c/source/test/testdata => testdata}/message2/syntax-errors-end-of-input.json (100%)
 rename {icu4c/source/test/testdata => testdata}/message2/tricky-declarations.json (100%)
 rename {icu4c/source/test/testdata => testdata}/message2/valid-tests.json (68%)

diff --git a/icu4c/source/config/dist.mk b/icu4c/source/config/dist.mk
index 6558c16477e5..a2750f99e8c3 100644
--- a/icu4c/source/config/dist.mk
+++ b/icu4c/source/config/dist.mk
@@ -68,6 +68,8 @@ $(DISTY_FILE_TGZ) $(DISTY_FILE_ZIP) $(DISTY_DATA_ZIP):  $(DISTY_DAT) $(DISTY_TMP
 	( cd $(ICU4CTOP)/.. && git archive --format=tar --prefix=icu/ HEAD:icu4c/ ) | ( cd "$(DISTY_TMP)" && tar xf - )
     # special handling for LICENSE file. The symlinks will be included as files by tar and zip.
 	cp -fv $(ICU4CTOP)/LICENSE "$(DISTY_TMP)/LICENSE"
+    # Copy top-level testdata directory so it's a sibling of the source/ directory
+	cp -R $(ICU4CTOP)/../testdata $(DISTY_TMP)/icu
 	( cd $(DISTY_TMP)/icu/source ; zip -rlq $(DISTY_DATA_ZIP) data )
 	$(MKINSTALLDIRS) $(DISTY_IN)
 	echo DISTY_DAT=$(DISTY_DAT)
diff --git a/icu4c/source/test/intltest/intltest.cpp b/icu4c/source/test/intltest/intltest.cpp
index 868dcbbd52d5..e46eaafca327 100644
--- a/icu4c/source/test/intltest/intltest.cpp
+++ b/icu4c/source/test/intltest/intltest.cpp
@@ -1679,6 +1679,61 @@ const char *IntlTest::getSourceTestData(UErrorCode& /*err*/) {
     return srcDataDir;
 }
 
+static bool fileExists(const char* fileName) {
+    // Test for `srcDataDir` existing by checking for `srcDataDir`/message2/valid-tests.json
+    U_ASSERT(fileName != nullptr);
+    FILE *f = fopen(fileName, "r");
+    if (f) {
+        fclose(f);
+        return true;
+    }
+    return false;
+}
+
+/**
+ * Returns the path to icu/testdata/
+ */
+const char *IntlTest::getSharedTestData(UErrorCode& err) {
+#define SOURCE_TARBALL_TOP U_TOPSRCDIR U_FILE_SEP_STRING ".." U_FILE_SEP_STRING
+#define REPO_TOP SOURCE_TARBALL_TOP ".." U_FILE_SEP_STRING
+#define FILE_NAME U_FILE_SEP_STRING "message2" U_FILE_SEP_STRING "valid-tests.json"
+    const char *srcDataDir = nullptr;
+    const char *testFile = nullptr;
+    if (U_SUCCESS(err)) {
+#ifdef U_TOPSRCDIR
+        // Try U_TOPSRCDIR/../testdata (source tarball)
+        srcDataDir = SOURCE_TARBALL_TOP "testdata" U_FILE_SEP_STRING;
+        testFile = SOURCE_TARBALL_TOP "testdata" FILE_NAME;
+        if (!fileExists(testFile)) {
+            // If that doesn't exist, try U_TOPSRCDIR/../../testdata (in-repo)
+            srcDataDir = REPO_TOP "testdata" U_FILE_SEP_STRING;
+            testFile = REPO_TOP "testdata" FILE_NAME;
+            if (!fileExists(testFile)) {
+                // If neither exists, return null
+                err = U_FILE_ACCESS_ERROR;
+                srcDataDir = nullptr;
+            }
+        }
+#else
+        // Try ../../../../testdata (if we're in icu/source/test/intltest)
+        // and ../../../../../../testdata (if we're in icu/source/test/intltest/Platform/(Debug|Release)
+#define TOP ".." U_FILE_SEP_STRING ".." U_FILE_SEP_STRING ".." U_FILE_SEP_STRING ".." U_FILE_SEP_STRING
+#define TOP_TOP ".." U_FILE_SEP_STRING ".." U_FILE_SEP_STRING TOP
+        srcDataDir = TOP "testdata" U_FILE_SEP_STRING;
+        testFile = TOP "testdata" FILE_NAME;
+        if (!fileExists(testFile)) {
+            srcDataDir = TOP_TOP "testdata" U_FILE_SEP_STRING;
+            testFile = TOP_TOP "testdata" FILE_NAME;
+            if (!fileExists(testFile)) {
+                err = U_FILE_ACCESS_ERROR;
+                srcDataDir = nullptr;
+            }
+        }
+#endif
+    }
+    return srcDataDir;
+}
+
 char *IntlTest::getUnidataPath(char path[]) {
     const int kUnicodeDataTxtLength = 15;  // strlen("UnicodeData.txt")
 
diff --git a/icu4c/source/test/intltest/intltest.h b/icu4c/source/test/intltest/intltest.h
index 146b7421bb5b..04cdb5c41c2b 100644
--- a/icu4c/source/test/intltest/intltest.h
+++ b/icu4c/source/test/intltest/intltest.h
@@ -420,6 +420,8 @@ class IntlTest : public TestLog {
     static const char* loadTestData(UErrorCode& err);
     virtual const char* getTestDataPath(UErrorCode& err) override;
     static const char* getSourceTestData(UErrorCode& err);
+    // Gets the path for the top-level testdata/ directory
+    static const char* getSharedTestData(UErrorCode& err);
     static char *getUnidataPath(char path[]);
     char16_t *ReadAndConvertFile(const char *fileName, int &ulen, const char *encoding, UErrorCode &status);
 
diff --git a/icu4c/source/test/intltest/messageformat2test_read_json.cpp b/icu4c/source/test/intltest/messageformat2test_read_json.cpp
index b52b3c48d4be..33e65a92ce85 100644
--- a/icu4c/source/test/intltest/messageformat2test_read_json.cpp
+++ b/icu4c/source/test/intltest/messageformat2test_read_json.cpp
@@ -39,7 +39,13 @@ static UErrorCode getExpectedErrorFromString(const std::string& errorName) {
 }
 
 static UErrorCode getExpectedRuntimeErrorFromString(const std::string& errorName) {
-    if (errorName == "unresolved-var") {
+    if (errorName == "parse-error" || errorName == "empty-token" || errorName == "extra-content") {
+        return U_MF_SYNTAX_ERROR;
+    }
+    if (errorName == "key-mismatch") {
+        return U_MF_VARIANT_KEY_MISMATCH_ERROR;
+    }
+    if (errorName == "missing-var" || errorName == "unresolved-var") {
         return U_MF_UNRESOLVED_VARIABLE_ERROR;
     }
     if (errorName == "unsupported-annotation") {
@@ -137,7 +143,8 @@ static void runValidTest(TestMessageFormat2& icuTest,
 
     // Certain ICU4J tests don't work yet in ICU4C.
     // See ICU-22754
-    if (!j_object["ignoreTest"].is_null()) {
+    // ignoreCpp => only works in Java
+    if (!j_object["ignoreCpp"].is_null()) {
         return;
     }
 
@@ -210,7 +217,7 @@ static void runSyntaxErrorTest(TestMessageFormat2& icuTest,
 static void runICU4JSyntaxTestsFromJsonFile(TestMessageFormat2& t,
                                             const std::string& fileName,
                                             IcuTestErrorCode& errorCode) {
-    const char* testDataDirectory = IntlTest::getSourceTestData(errorCode);
+    const char* testDataDirectory = IntlTest::getSharedTestData(errorCode);
     CHECK_ERROR(errorCode);
 
     std::string testFileName(testDataDirectory);
@@ -252,7 +259,7 @@ static void runICU4JSyntaxTestsFromJsonFile(TestMessageFormat2& t,
 static void runICU4JSelectionTestsFromJsonFile(TestMessageFormat2& t,
                                             const std::string& fileName,
                                             IcuTestErrorCode& errorCode) {
-    const char* testDataDirectory = IntlTest::getSourceTestData(errorCode);
+    const char* testDataDirectory = IntlTest::getSharedTestData(errorCode);
     CHECK_ERROR(errorCode);
 
     std::string testFileName(testDataDirectory);
@@ -270,7 +277,7 @@ static void runICU4JSelectionTestsFromJsonFile(TestMessageFormat2& t,
         auto variations = j_object["variations"];
 
         // Skip ignored tests
-        if (!j_object["ignoreTest"].is_null()) {
+        if (!j_object["ignoreCpp"].is_null()) {
             return;
         }
 
@@ -281,7 +288,7 @@ static void runICU4JSelectionTestsFromJsonFile(TestMessageFormat2& t,
             messageText += piece;
         }
 
-        t.logln("ICU4J selectors tests:");
+        t.logln(u_str("ICU4J selectors tests: " + fileName));
         t.logln(u_str(iter->dump()));
 
         TestCase::Builder test;
@@ -315,7 +322,7 @@ static void runICU4JSelectionTestsFromJsonFile(TestMessageFormat2& t,
 static void runValidTestsFromJsonFile(TestMessageFormat2& t,
                                       const std::string& fileName,
                                       IcuTestErrorCode& errorCode) {
-    const char* testDataDirectory = IntlTest::getSourceTestData(errorCode);
+    const char* testDataDirectory = IntlTest::getSharedTestData(errorCode);
     CHECK_ERROR(errorCode);
 
     std::string testFileName(testDataDirectory);
@@ -341,7 +348,7 @@ static void runValidTestsFromJsonFile(TestMessageFormat2& t,
 static void runDataModelErrorTestsFromJsonFile(TestMessageFormat2& t,
                                                const std::string& fileName,
                                                IcuTestErrorCode& errorCode) {
-    const char* testDataDirectory = IntlTest::getSourceTestData(errorCode);
+    const char* testDataDirectory = IntlTest::getSharedTestData(errorCode);
     CHECK_ERROR(errorCode);
 
     std::string dataModelErrorsFileName(testDataDirectory);
@@ -368,7 +375,7 @@ static void runDataModelErrorTestsFromJsonFile(TestMessageFormat2& t,
         for (auto messagesIter = messages.begin(); messagesIter != messages.end(); ++messagesIter) {
             makeTestName(testName, sizeof(testName), errorName, testNum);
             testBuilder.setName(testName);
-            t.logln(testName);
+            t.logln(u_str(fileName + ": " + testName));
             testNum++;
             UnicodeString messageText = u_str(*messagesIter);
             t.logln(messageText);
@@ -386,7 +393,7 @@ static void runDataModelErrorTestsFromJsonFile(TestMessageFormat2& t,
 static void runSyntaxErrorTestsFromJsonFile(TestMessageFormat2& t,
                                             const std::string& fileName,
                                             IcuTestErrorCode& errorCode) {
-    const char* testDataDirectory = IntlTest::getSourceTestData(errorCode);
+    const char* testDataDirectory = IntlTest::getSharedTestData(errorCode);
     CHECK_ERROR(errorCode);
 
     std::string syntaxErrorsFileName(testDataDirectory);
@@ -420,7 +427,7 @@ static void runSyntaxErrorTestsFromJsonFile(TestMessageFormat2& t,
 static void runSyntaxTestsWithDiagnosticsFromJsonFile(TestMessageFormat2& t,
                                                       const std::string& fileName,
                                                       IcuTestErrorCode& errorCode) {
-    const char* testDataDirectory = IntlTest::getSourceTestData(errorCode);
+    const char* testDataDirectory = IntlTest::getSharedTestData(errorCode);
     CHECK_ERROR(errorCode);
 
     std::string testFileName(testDataDirectory);
@@ -446,7 +453,7 @@ static void runFunctionTestsFromJsonFile(TestMessageFormat2& t,
                                          const std::string& fileName,
                                          IcuTestErrorCode& errorCode) {
     // Get the test data directory
-    const char* testDataDirectory = IntlTest::getSourceTestData(errorCode);
+    const char* testDataDirectory = IntlTest::getSharedTestData(errorCode);
     CHECK_ERROR(errorCode);
 
     std::string functionTestsFileName(testDataDirectory);
@@ -460,7 +467,7 @@ static void runFunctionTestsFromJsonFile(TestMessageFormat2& t,
     for (auto iter = tests.begin(); iter != tests.end(); ++iter) {
         int32_t testNum = 0;
         auto functionName = iter->first;
-        t.logln("Function tests:");
+        t.logln(u_str("Function tests: " + fileName));
         t.logln(u_str(iter->second.dump()));
 
         // Array of tests
@@ -487,6 +494,7 @@ void TestMessageFormat2::jsonTestsFromFiles(IcuTestErrorCode& errorCode) {
 
     // Do spec tests for syntax errors
     runSyntaxErrorTestsFromJsonFile(*this, "spec/syntax-errors.json", errorCode);
+    runSyntaxErrorTestsFromJsonFile(*this, "more-syntax-errors.json", errorCode);
 
     // Do tests for data model errors
     runDataModelErrorTestsFromJsonFile(*this, "spec/data-model-errors.json", errorCode);
@@ -540,13 +548,10 @@ void TestMessageFormat2::jsonTestsFromFiles(IcuTestErrorCode& errorCode) {
     runSyntaxTestsWithDiagnosticsFromJsonFile(*this, "syntax-errors-diagnostics-multiline.json", errorCode);
 
     // ICU4J tests
-    runFunctionTestsFromJsonFile(*this, "icu4j/icu-test-functions.json", errorCode);
-    // Changes made to get these tests to work:
-    // * removed double backslashes from  "Hello \\t \\n \\r \\{ world!"
-    // * added empty {{}} pattern at the end of the tests under "Simple messages, with declarations"
-    //   and the first one under "Multiple declarations in one message"
-    runICU4JSyntaxTestsFromJsonFile(*this, "icu4j/icu-parser-tests.json", errorCode);
-    runICU4JSelectionTestsFromJsonFile(*this, "icu4j/icu-test-selectors.json", errorCode);
+    runFunctionTestsFromJsonFile(*this, "icu-test-functions.json", errorCode);
+    runICU4JSyntaxTestsFromJsonFile(*this, "icu-parser-tests.json", errorCode);
+    runICU4JSelectionTestsFromJsonFile(*this, "icu-test-selectors.json", errorCode);
+    runValidTestsFromJsonFile(*this, "icu-test-previous-release.json", errorCode);
 }
 
 #endif /* #if !UCONFIG_NO_MF2 */
diff --git a/icu4c/source/test/testdata/message2/README.txt b/icu4c/source/test/testdata/message2/README.txt
deleted file mode 100644
index 65996cab7bde..000000000000
--- a/icu4c/source/test/testdata/message2/README.txt
+++ /dev/null
@@ -1,40 +0,0 @@
-© 2024 and later: Unicode, Inc. and others.
-License & terms of use: http://www.unicode.org/copyright.html
-
-The format of the JSON files in this directory follows the same format as `test-core.json`
-in the spec, described in:
-
-https://github.com/unicode-org/message-format-wg/blob/main/test/README.md
-
-The `parts` and `cleanSrc` fields are not used.
-
-Some extensions:
-
-Additional "char" and "line" fields may be present with integer values,
-used for tests expected to trigger a syntax error.
-If present, "char" reflects the expected character offset and "line"
-reflects the expected line number in the parse error.
-The files with "diagnostics" in the name have these fields filled in.
-
-An additional `comment` field may be present, which is only for human readers.
-
-A "srcs" field, whose value is an array of strings, may be present instead
-of "src". The strings are concatenated to get the message.
-
-In the "params" field, a date parameter can be expressed as:
-{ "date": n }
-where n is a number representing a Unix timestamp.
-
-An optional field, "ignoreTest", can be used for tests that are
-currently expected to fail. The field may have any value; if it's
-present, the test is ignored. (The value can be a comment explaining
-why it's expected to fail.)
-
-Tests in the `icu4j/` subdirectory are taken from:
- icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2
- and need to be manually synced with those files. The format is a bit
- different in some cases.
-
-Tests in the `spec/` subdirectory are taken from https://github.com/unicode-org/message-format-wg/blob/main/test
-and need to be manually updated if the contents change upstream.
-
diff --git a/icu4c/source/test/testdata/message2/invalid-options.json b/icu4c/source/test/testdata/message2/invalid-options.json
deleted file mode 100644
index 0ae458375be7..000000000000
--- a/icu4c/source/test/testdata/message2/invalid-options.json
+++ /dev/null
@@ -1,10 +0,0 @@
-[
-    { "src": ".local $foo = {1 :number minimumIntegerDigits=-1} {{bar {$foo}}}", "errors": [{"type": "bad-option"}]},
-    { "src": ".local $foo = {1 :number minimumIntegerDigits=foo} {{bar {$foo}}}", "errors": [{"type": "bad-option"}]},
-    { "src": ".local $foo = {1 :number minimumFractionDigits=foo} {{bar {$foo}}}", "errors": [{"type": "bad-option"}]},
-    { "src": ".local $foo = {1 :number maximumFractionDigits=foo} {{bar {$foo}}}", "errors": [{"type": "bad-option"}]},
-    { "src": ".local $foo = {1 :number minimumSignificantDigits=foo} {{bar {$foo}}}", "errors": [{"type": "bad-option"}]},
-    { "src": ".local $foo = {1 :number maximumSignificantDigits=foo} {{bar {$foo}}}", "errors": [{"type": "bad-option"}]},
-    { "src": ".local $foo = {1 :integer minimumIntegerDigits=foo} {{bar {$foo}}}", "errors": [{"type": "bad-option"}]},
-    { "src": ".local $foo = {1 :integer maximumSignificantDigits=foo} {{bar {$foo}}}", "errors": [{"type": "bad-option"}]}
-]
diff --git a/icu4c/source/test/testdata/message2/reserved-syntax.json b/icu4c/source/test/testdata/message2/reserved-syntax.json
deleted file mode 100644
index 29397f54cc27..000000000000
--- a/icu4c/source/test/testdata/message2/reserved-syntax.json
+++ /dev/null
@@ -1,39 +0,0 @@
-[
-    { "src": "hello {|4.2| %number}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| %n|um|ber}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "{+42}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| &num|be|r}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| ^num|be|r}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| +num|be|r}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| ?num|be||r|s}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|foo| !number}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|foo| *number}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {?number}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "{<tag}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": ".local $bar = {$none ~plural} .match {foo :string}  * {{{$bar}}}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| %num\\\\ber}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| %num\\{be\\|r}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| %num\\\\\\}ber}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| !}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| %}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| *}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| ^abc|123||5|\\\\}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| ^ abc|123||5|\\\\}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| ^ abc|123||5|\\\\ \\|def |3.14||2|}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| ? }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| %xyzz }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| >xyzz   }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {$foo ~xyzz }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {$x   <xyzz   }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "{>xyzz }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "{  !xyzz   }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "{~xyzz }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "{ <xyzz   }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| !xy z z }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| *num \\\\ b er}", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2| %num \\\\ b |3.14| r    }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {|4.2|    +num xx \\\\ b |3.14| r  }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {$foo    +num x \\\\ abcde |3.14| r  }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {$foo    >num x \\\\ abcde |aaa||3.14||42| r  }", "errors": [{ "type": "unsupported-annotation" }] },
-    { "src": "hello {$foo    >num x \\\\ abcde |aaa||3.14| |42| r  }", "errors": [{ "type": "unsupported-annotation" }] }
-]
diff --git a/icu4c/source/test/testdata/message2/resolution-errors.json b/icu4c/source/test/testdata/message2/resolution-errors.json
deleted file mode 100644
index 82595b4a27ce..000000000000
--- a/icu4c/source/test/testdata/message2/resolution-errors.json
+++ /dev/null
@@ -1,8 +0,0 @@
-[
-    { "src": "{$oops}", "exp": "{$oops}", "errors": [{ "type": "unresolved-var" }]},
-    { "src": ".input {$x :number} {{{$x}}}", "exp": "{$x}", "errors": [{ "type": "unresolved-var" }]},
-    { "src": ".local $foo = {$bar} .match {$foo :number}  one {{one}}  * {{other}}", "exp": "other", "errors": [{ "type": "unresolved-var" }]},
-    { "src": ".local $foo = {$bar} .match {$foo :number}  one {{one}}  * {{other}}", "exp": "other", "errors": [{ "type": "unresolved-var" }]},
-    { "src": ".local $bar = {$none :number} .match {$foo :string}  one {{one}}  * {{{$bar}}}", "exp": "{$none}", "errors": [{ "type": "unresolved-var" }]},
-    { "src": "The value is {horse :func}.", "exp": "The value is {|horse|}.", "errors": [{ "type": "missing-func" }]}
-]
diff --git a/icu4c/source/test/testdata/message2/spec/test-functions.json b/icu4c/source/test/testdata/message2/spec/test-functions.json
deleted file mode 100644
index 3258d2a5b39f..000000000000
--- a/icu4c/source/test/testdata/message2/spec/test-functions.json
+++ /dev/null
@@ -1,306 +0,0 @@
-{
-  "date": [
-    { "src": "{:date}", "exp": "{:date}", "errors": [{ "type": "bad-input" }] },
-    {
-      "src": "{horse :date}",
-      "exp": "{|horse|}",
-      "errors": [{ "type": "bad-input" }]
-    },
-    { "src": "{|2006-01-02| :date}", "exp": "1/2/06" },
-    { "src": "{|2006-01-02T15:04:06| :date}", "exp": "1/2/06" },
-    { "src": "{|2006-01-02| :date style=long}", "exp": "January 2, 2006" },
-    {
-      "src": ".local $d = {|2006-01-02| :date style=long} {{{$d :date}}}",
-      "exp": "January 2, 2006"
-    },
-    {
-      "src": ".local $t = {|2006-01-02T15:04:06| :time} {{{$t :date}}}",
-      "exp": "1/2/06"
-    }
-  ],
-  "time": [
-    { "src": "{:time}", "exp": "{:time}", "errors": [{ "type": "bad-input" }] },
-    {
-      "src": "{horse :time}",
-      "exp": "{|horse|}",
-      "errors": [{ "type": "bad-input" }]
-    },
-    { "src": "{|2006-01-02T15:04:06| :time}", "exp": "3:04\u202FPM" },
-    {
-      "src": "{|2006-01-02T15:04:06| :time style=medium}",
-      "exp": "3:04:06\u202FPM"
-    },
-    {
-      "src": ".local $t = {|2006-01-02T15:04:06| :time style=medium} {{{$t :time}}}",
-      "exp": "3:04:06\u202FPM"
-    },
-    {
-      "src": ".local $d = {|2006-01-02T15:04:06| :date} {{{$d :time}}}",
-      "exp": "3:04\u202FPM"
-    }
-  ],
-  "datetime": [
-    {
-      "src": "{:datetime}",
-      "exp": "{:datetime}",
-      "errors": [{ "type": "bad-input" }]
-    },
-    {
-      "src": "{$x :datetime}",
-      "exp": "{$x}",
-      "params": { "x": true },
-      "errors": [{ "type": "bad-input" }]
-    },
-    {
-      "src": "{horse :datetime}",
-      "exp": "{|horse|}",
-      "errors": [{ "name": "RangeError" }]
-    },
-    { "src": "{|2006-01-02T15:04:06| :datetime}", "exp": "1/2/06, 3:04\u202FPM" },
-    {
-      "src": "{|2006-01-02T15:04:06| :datetime year=numeric month=|2-digit|}",
-      "exp": "01/2006"
-    },
-    {
-      "src": "{|2006-01-02T15:04:06| :datetime dateStyle=long}",
-      "exp": "January 2, 2006"
-    },
-    {
-      "src": "{|2006-01-02T15:04:06| :datetime timeStyle=medium}",
-      "exp": "3:04:06\u202FPM"
-    },
-    {
-      "src": "{$dt :datetime}",
-      "params": { "dt": "2006-01-02T15:04:06" },
-      "exp": "1/2/06, 3:04\u202FPM"
-    }
-  ],
-  "integer": [
-    { "src": "hello {4.2 :integer}", "exp": "hello 4" },
-    { "src": "hello {-4.20 :integer}", "exp": "hello -4" },
-    { "src": "hello {0.42e+1 :integer}", "exp": "hello 4" },
-    {
-      "src": ".match {$foo :integer} one {{one}} * {{other}}",
-      "params": { "foo": 1.2 },
-      "exp": "one"
-    }
-  ],
-  "number": [
-    { "src": "hello {4.2 :number}", "exp": "hello 4.2" },
-    { "src": "hello {-4.20 :number}", "exp": "hello -4.2" },
-    { "src": "hello {0.42e+1 :number}", "exp": "hello 4.2" },
-    {
-      "src": "hello {foo :number}",
-      "exp": "hello {|foo|}",
-      "errors": [{ "type": "bad-input" }]
-    },
-    {
-      "src": "hello {:number}",
-      "exp": "hello {:number}",
-      "errors": [{ "type": "bad-input" }]
-    },
-    {
-      "src": "hello {4.2 :number minimumFractionDigits=2}",
-      "exp": "hello 4.20"
-    },
-    {
-      "src": "hello {|4.2| :number minimumFractionDigits=|2|}",
-      "exp": "hello 4.20"
-    },
-    {
-      "src": "hello {4.2 :number minimumFractionDigits=$foo}",
-      "params": { "foo": 2 },
-      "exp": "hello 4.20"
-    },
-    {
-      "src": "hello {|4.2| :number minimumFractionDigits=$foo}",
-      "params": { "foo": "2" },
-      "exp": "hello 4.20"
-    },
-    {
-      "src": ".local $foo = {$bar :number} {{bar {$foo}}}",
-      "params": { "bar": 4.2 },
-      "exp": "bar 4.2"
-    },
-    {
-      "src": ".local $foo = {$bar :number minimumFractionDigits=2} {{bar {$foo}}}",
-      "params": { "bar": 4.2 },
-      "exp": "bar 4.20"
-    },
-    {
-      "src": ".local $foo = {$bar :number minimumFractionDigits=foo} {{bar {$foo}}}",
-      "params": { "bar": 4.2 },
-      "exp": "bar {$bar}",
-      "errors": [{ "type": "bad-option" }]
-    },
-    {
-      "src": ".local $foo = {$bar :number} {{bar {$foo}}}",
-      "params": { "bar": "foo" },
-      "exp": "bar {$bar}",
-      "errors": [{ "type": "bad-input" }]
-    },
-    {
-      "src": ".input {$foo :number} {{bar {$foo}}}",
-      "params": { "foo": 4.2 },
-      "exp": "bar 4.2"
-    },
-    {
-      "src": ".input {$foo :number minimumFractionDigits=2} {{bar {$foo}}}",
-      "params": { "foo": 4.2 },
-      "exp": "bar 4.20"
-    },
-    {
-      "src": ".input {$foo :number minimumFractionDigits=foo} {{bar {$foo}}}",
-      "params": { "foo": 4.2 },
-      "exp": "bar {$foo}",
-      "errors": [{ "type": "bad-option" }]
-    },
-    {
-      "src": ".input {$foo :number} {{bar {$foo}}}",
-      "params": { "foo": "foo" },
-      "exp": "bar {$foo}",
-      "errors": [{ "type": "bad-input" }]
-    },
-    {
-      "src": ".match {$foo :number} one {{one}} * {{other}}",
-      "params": { "foo": 1 },
-      "exp": "one"
-    },
-    {
-      "src": ".match {$foo :number} 1 {{=1}} one {{one}} * {{other}}",
-      "params": { "foo": 1 },
-      "exp": "=1"
-    },
-    {
-      "src": ".match {$foo :number} one {{one}} 1 {{=1}} * {{other}}",
-      "params": { "foo": 1 },
-      "exp": "=1"
-    },
-    {
-      "src": ".match {$foo :number} {$bar :number} one one {{one one}} one * {{one other}} * * {{other}}",
-      "params": { "foo": 1, "bar": 1 },
-      "exp": "one one"
-    },
-    {
-      "src": ".match {$foo :number} {$bar :number} one one {{one one}} one * {{one other}} * * {{other}}",
-      "params": { "foo": 1, "bar": 2 },
-      "exp": "one other"
-    },
-    {
-      "src": ".match {$foo :number} {$bar :number} one one {{one one}} one * {{one other}} * * {{other}}",
-      "params": { "foo": 2, "bar": 2 },
-      "exp": "other"
-    },
-    {
-      "src": ".input {$foo :number} .match {$foo} one {{one}} * {{other}}",
-      "params": { "foo": 1 },
-      "exp": "one"
-    },
-    {
-      "src": ".local $foo = {$bar :number} .match {$foo} one {{one}} * {{other}}",
-      "params": { "bar": 1 },
-      "exp": "one"
-    },
-    {
-      "src": ".input {$foo :number} .local $bar = {$foo} .match {$bar} one {{one}} * {{other}}",
-      "params": { "foo": 1 },
-      "exp": "one"
-    },
-    {
-      "src": ".input {$bar :number} .match {$bar} one {{one}} * {{other}}",
-      "params": { "bar": 2 },
-      "exp": "other"
-    },
-    {
-      "src": ".input {$bar} .match {$bar :number} one {{one}} * {{other}}",
-      "params": { "bar": 1 },
-      "exp": "one"
-    },
-    {
-      "src": ".input {$bar} .match {$bar :number} one {{one}} * {{other}}",
-      "params": { "bar": 2 },
-      "exp": "other"
-    },
-    {
-      "src": ".input {$bar} .match {$bar :number} one {{one}} * {{other}}",
-      "params": { "bar": 1 },
-      "exp": "one"
-    },
-    {
-      "src": ".input {$bar} .match {$bar :number} one {{one}} * {{other}}",
-      "params": { "bar": 2 },
-      "exp": "other"
-    },
-    {
-      "src": ".input {$none} .match {$foo :number} one {{one}} * {{{$none}}}",
-      "params": { "foo": 1 },
-      "exp": "one"
-    },
-    {
-      "src": ".local $bar = {$none} .match {$foo :number} one {{one}} * {{{$bar}}}",
-      "params": { "foo": 1 },
-      "exp": "one"
-    },
-    {
-      "src": ".local $bar = {$none} .match {$foo :number} one {{one}} * {{{$bar}}}",
-      "params": { "foo": 2 },
-      "exp": "{$none}",
-      "errors": [{ "type": "unresolved-var" }]
-    },
-    {
-      "src": "{42 :number @foo @bar=13}",
-      "exp": "42",
-      "parts": [
-        { "type": "number", "parts": [{ "type": "integer", "value": "42" }] }
-      ]
-    }
-  ],
-  "ordinal": [
-    {
-      "src": ".match {$foo :ordinal} one {{st}} two {{nd}} few {{rd}} * {{th}}",
-      "params": { "foo": 1 },
-      "exp": "th",
-      "errors": [{ "type": "missing-func" }, { "type": "not-selectable" }]
-    },
-    {
-      "src": "hello {42 :ordinal}",
-      "exp": "hello {|42|}",
-      "errors": [{ "type": "missing-func" }]
-    }
-  ],
-  "plural": [
-    {
-      "src": ".match {$foo :plural} one {{one}} * {{other}}",
-      "params": { "foo": 1 },
-      "exp": "other",
-      "errors": [{ "type": "missing-func" }, { "type": "not-selectable" }]
-    },
-    {
-      "src": "hello {42 :plural}",
-      "exp": "hello {|42|}",
-      "errors": [{ "type": "missing-func" }]
-    }
-  ],
-  "string": [
-    {
-      "src": ".match {$foo :string} |1| {{one}} * {{other}}",
-      "params": { "foo": "1" },
-      "exp": "one"
-    },
-    {
-      "src": ".match {$foo :string} 1 {{one}} * {{other}}",
-      "params": { "foo": 1 },
-      "exp": "one"
-    },
-    {
-      "src": ".match {$foo :string} 1 {{one}} * {{other}}",
-      "params": { "foo": null },
-      "exp": "other"
-    },
-    {
-      "src": ".match {$foo :string} 1 {{one}} * {{other}}",
-      "exp": "other",
-      "errors": [{ "type": "unresolved-var" }]
-    }
-  ]
-}
diff --git a/icu4c/source/test/testdata/message2/syntax-errors-diagnostics.json b/icu4c/source/test/testdata/message2/syntax-errors-diagnostics.json
deleted file mode 100644
index b4b86c68e8e7..000000000000
--- a/icu4c/source/test/testdata/message2/syntax-errors-diagnostics.json
+++ /dev/null
@@ -1,352 +0,0 @@
-[
-    { "src": "}{|xyz|", "char": 0 },
-    { "src": "}", "char": 0 },
-    {
-        "src": "{{{%\\y{}}",
-        "char": 5,
-        "comment": "Backslash followed by non-backslash followed by a '{' -- this should be an error immediately after the first backslash"
-    },
-    {
-        "src": "{%abc|\\z}}",
-        "char": 7,
-        "comment": "Reserved chars followed by a '|' that doesn't begin a valid literal -- this should be an error at the first invalid char in the literal"
-    },
-    {
-        "src": "{%\\y{p}}",
-        "char": 3,
-        "comment": "Same pattern, but with a valid reserved-char following the erroneous reserved-escape -- the offset should be the same as with the previous one"
-    },
-    {
-        "src": "{{{%ab|\\z|cd}}",
-        "char": 8,
-        "comment": "Erroneous literal inside a reserved string -- the error should be at the first erroneous literal char"
-    },
-    {
-        "src": "hello {|4.2| %num\\ber}}",
-        "char": 18,
-        "comment": "Single backslash not allowed"
-    },
-    {
-        "src": "hello {|4.2| %num{be\\|r}}",
-        "char": 17,
-        "comment": "Unescaped '{' not allowed"
-    },
-    {
-        "src": "hello {|4.2| %num}be\\|r}}",
-        "char": 21,
-        "comment": "Unescaped '}' -- will be interpreted as the end of the reserved string, and the error will be reported at the index of '|', which is when the parser determines that the escaped '|' isn't a valid text-escape"
-    },
-    {
-        "src": "hello {|4.2| %num\\{be|r}}",
-        "char": 25,
-        "comment": "Unescaped '|' -- will be interpreted as the beginning of a literal. Error at end of input"
-    },
-    {
-        "src": "a\\qbc",
-        "char": 2,
-        "comment": "Invalid escape sequence in a `text` -- the error should be at the character following the backslash"
-    },
-    {
-        "src": ".match{|y|}|y|{{{|||}}}",
-        "char": 19,
-        "comment": "No spaces are required here. The error should be in the pattern, not before"
-    },
-    {
-        "src": ".match {|y|}|foo|bar {{{a}}}",
-        "char": 17,
-        "comment": "Missing spaces between keys"
-    },
-    {
-        "src": ".match {|y|} |quux| |foo|bar {{{a}}}",
-        "char": 25,
-        "comment": "Missing spaces between keys"
-    },
-    {
-        "src": ".match {|y|}  |quux| |foo||bar| {{{a}}}",
-        "char": 26,
-        "comment": "Missing spaces between keys"
-    },
-    {
-        "src": ".match {|y|}  |\\q| * %{! {z}",
-        "char": 16,
-        "comment": "Error parsing the first key -- the error should be there, not in the also-erroneous third key"
-    },
-    {
-        "src":  ".match {|y|}  * %{! {z} |\\q|",
-        "char": 16,
-        "comment": "Error parsing the second key -- the error should be there, not in the also-erroneous third key"
-    },
-    {
-        "src": ".match {|y|}  * |\\q| {\\z}",
-        "char": 18,
-        "comment": "Error parsing the last key -- the error should be there, not in the erroneous pattern"
-    },
-    {
-        "src": ".match {|y|} {\\|} {@}  * * * {{a}}",
-        "char": 14,
-        "comment": "Non-expression as scrutinee in pattern -- error should be at the first non-expression, not the later non-expression"
-    },
-    {
-        "src": ".match {|y|}  $foo * {{a}} when * :bar {{b}}",
-        "char": 14,
-        "comment": "Non-key in variant -- error should be there, not in the next erroneous variant"
-    },
-    {
-        "src": "{{ foo {|bar|} \\q baz  ",
-        "char": 16,
-        "comment": "Error should be within the first erroneous `text` or expression"
-    },
-    {
-        "src": "{{{:    }}}",
-        "char": 4,
-        "comment": "':' has to be followed by a function name -- the error should be at the first whitespace character"
-    },
-    {
-        "src": ".local $x = }|foo|}",
-        "char": 12,
-        "comment": "Expression not starting with a '{'"
-    },
-    {
-        "src": ".local $x = {|foo|} .l $y = {|bar|} .local $z {|quux|}",
-        "char": 22,
-        "comment": "Error should be at the first declaration not starting with a `.local`"
-    },
-    {
-        "src": ".local $bar {|foo|} {{$bar}}",
-        "char": 12,
-        "comment": "Missing '=' in `.local` declaration"
-    },
-    {
-        "src": ".local bar = {|foo|} {{$bar}}",
-        "char": 7,
-        "comment": "LHS of declaration doesn't start with a '$'"
-    },
-    {
-        "src": ".local $bar = |foo| {{$bar}}",
-        "char": 14,
-        "comment": "`.local` RHS isn't an expression"
-    },
-    {
-        "src": "{{extra}}content",
-        "char": 9,
-        "comment": "Trailing characters that are not whitespace"
-    },
-    {
-        "src": ".match {|x|}  * {{foo}}extra",
-        "char": 28,
-        "comment": "Trailing characters that are not whitespace"
-    },
-    {
-        "src": ".match {$foo :string} {$bar :string}  one * {{one}}  * * {{other}}   ",
-        "char": 66,
-        "comment": "Trailing whitespace at end of message should not be accepted either"
-    },
-    {
-        "src": "{{hi}} ",
-        "char": 6,
-        "comment": "Trailing whitespace at end of message should not be accepted either"
-    },
-    {
-        "src": "empty { }",
-        "char": 8,
-        "comment": "Empty expression"
-    },
-    {
-        "src": ".match {}  * {{foo}}",
-        "char": 8,
-        "comment": "Empty expression"
-    },
-    {
-        "src": "bad {:}",
-        "char": 6,
-        "comment": "':' not preceding a function name"
-    },
-    {
-        "src": "{{no-equal {|42| :number m }}}",
-        "char":  27,
-        "comment": "Missing '=' after option name"
-    },
-    {
-        "src": "{{no-equal {|42| :number minimumFractionDigits 2}}}",
-        "char":  47,
-        "comment": "Missing '=' after option name"
-    },
-    {
-        "src": "bad {:placeholder option value}",
-        "char":  25,
-        "comment": "Missing '=' after option name"
-    },
-    {
-        "src": "hello {|4.2| :number min=2=3}",
-        "char":  26,
-        "comment": "Extra '=' after option name"
-    },
-    {
-        "src": "hello {|4.2| :number min=2max=3}",
-        "char":  26,
-        "comment": "Extra '=' after option name"
-    },
-    {
-        "src": "hello {|4.2| :number min=|a|max=3}",
-        "char": 28,
-        "comment": "Missing whitespace between valid options"
-    },
-    {
-        "src": "hello {|4.2| :number min=|\\a|}",
-        "char": 27,
-        "comment": "Ill-formed RHS of option -- the error should be within the RHS, not after parsing options"
-    },
-    {
-        "src": "no-equal {|42| :number   {}",
-        "char": 25,
-        "comment": "Junk after annotation"
-    },
-    {
-        "src": "bad {:placeholder option=}",
-        "char": 25,
-        "comment": "Missing RHS of option"
-    },
-    {
-        "src": "bad {:placeholder option}",
-        "char": 24,
-        "comment": "Missing RHS of option"
-    },
-    {
-        "src": "bad {$placeholder option}",
-        "char": 18,
-        "comment": "Annotation is not a function or reserved text"
-    },
-    {
-        "src": "no {$placeholder end",
-        "char": 17,
-        "comment": "Annotation is not a function or reserved text"
-    },
-    {
-        "src": ".match  * {{foo}}",
-        "char": 8,
-        "comment": "Missing expression in selectors"
-    },
-    {
-        "src": ".match |x|  * {{foo}}",
-        "char": 7,
-        "comment": "Non-expression in selectors"
-    },
-    {
-        "src": ".match {|x|}  * foo",
-        "char": 19,
-        "comment": "Missing RHS in variant"
-    },
-    {
-        "src": "{$:abc}",
-        "char": 2,
-        "comment": "Variable names can't start with a : or -"
-    },
-    {
-        "src": "{$-abc}",
-        "char": 2,
-        "comment": "Variable names can't start with a : or -"
-    },
-    {
-        "src": "{$bar+foo}",
-        "char": 5,
-        "comment": "Missing space before annotation. Note that {{$bar:foo}} and {{$bar-foo}} are valid, because variable names can contain a ':' or a '-'"
-    },
-    {
-        "src": "{|3.14|:foo}",
-        "char": 7,
-        "comment": "Missing space before annotation."
-    },
-    {
-        "src": "{|3.14|-foo}",
-        "char": 7,
-        "comment": "Missing space before annotation."
-    },
-    {
-        "src": "{|3.14|+foo}",
-        "char": 7,
-        "comment": "Missing space before annotation."
-    },
-    {
-        "src": ".local $foo = {$bar} .match {$foo}  :one {one} * {other}",
-        "char": 36,
-        "comment": "Unquoted literals can't begin with a ':'"
-    },
-    {
-        "src": ".local $foo = {$bar :fun option=:a} {{bar {$foo}}}",
-        "char": 32,
-        "comment": "Unquoted literals can't begin with a ':'"
-    },
-    { "src": "{|foo| {#markup}}", "char": 7, "comment": "Markup in wrong place" },
-    { "src": "{|foo| #markup}", "char": 7, "comment": "Markup in wrong place" },
-    { "src": "{|foo| {#markup/}}", "char": 7, "comment": "Markup in wrong place" },
-    { "src": "{|foo| {/markup}}", "char": 7, "comment": "Markup in wrong place" },
-    { "src": ".input $x = {|1|} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression" },
-    { "src": ".input $x = {:number} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression" },
-    { "src": ".input {|1| :number} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression" },
-    { "src": ".input {:number} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression" },
-    { "src": ".input {|1|} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression" },
-    { "src": ".", "char": 1},
-    { "src":  "{", "char": 1},
-    { "src":  "}", "char": 0},
-    { "src":  "{}", "char": 1},
-    { "src":  "{{", "char": 2},
-    { "src":  "{{}", "char": 3},
-    { "src":  "{{}}}", "char": 4},
-    { "src":  "{|foo| #markup}", "char": 7},
-    { "src":  "{{missing end brace}", "char": 20},
-    { "src":  "{{missing end braces", "char": 20},
-    { "src":  "{{missing end {$braces", "char": 22},
-    { "src":  "{{extra}} content", "char": 9},
-    { "src":  "empty { } placeholder", "char": 8},
-    { "src":  "missing space {42:func}", "char": 17},
-    { "src":  "missing space {|foo|:func}", "char": 20},
-    { "src":  "missing space {|foo|@bar}", "char": 20},
-    { "src":  "missing space {:func@bar}", "char": 20},
-    { "src":  "{:func @bar@baz}", "char": 11},
-    { "src":  "{:func @bar=42@baz}", "char": 14},
-    { "src":  "{+reserved@bar}", "char": 10},
-    { "src":  "{&private@bar}", "char": 9},
-    { "src":  "bad {:} placeholder", "char": 6},
-    { "src":  "bad {\\u0000placeholder}", "char": 5},
-    { "src":  "no-equal {|42| :number minimumFractionDigits 2}", "char": 45},
-    { "src":  "bad {:placeholder option=}", "char": 25},
-    { "src":  "bad {:placeholder option value}", "char": 25},
-    { "src":  "bad {:placeholder option:value}", "char": 30},
-    { "src":  "bad {:placeholder option}", "char": 24},
-    { "src":  "bad {:placeholder:}", "char": 18},
-    { "src":  "bad {::placeholder}", "char": 6},
-    { "src":  "bad {:placeholder::foo}", "char": 18},
-    { "src":  "bad {:placeholder option:=x}", "char": 25},
-    { "src":  "bad {:placeholder :option=x}", "char": 18},
-    { "src":  "bad {:placeholder option::x=y}", "char": 25},
-    { "src":  "bad {$placeholder option}", "char": 18},
-    { "src":  "bad {:placeholder @attribute=}", "char": 29},
-    { "src":  "bad {:placeholder @attribute=@foo}", "char": 29},
-    { "src":  "no {placeholder end", "char": 16},
-    { "src":  "no {$placeholder end", "char": 17},
-    { "src":  "no {:placeholder end", "char": 20},
-    { "src":  "no {|placeholder| end", "char": 18},
-    { "src":  "no {|literal} end", "char": 17},
-    { "src":  "no {|literal or placeholder end", "char": 31},
-    { "src":  ".local bar = {|foo|} {{_}}", "char": 7},
-    { "src":  ".local #bar = {|foo|} {{_}}", "char": 7},
-    { "src":  ".local $bar {|foo|} {{_}}", "char": 12},
-    { "src":  ".local $bar = |foo| {{_}}", "char": 14},
-    { "src":  ".match {#foo} * {{foo}}", "char": 8},
-    { "src":  ".match {} * {{foo}}", "char": 8},
-    { "src":  ".match {|foo| :x} {|bar| :x} ** {{foo}}", "char": 30},
-    { "src":  ".match * {{foo}}", "char": 7},
-    { "src":  ".match {|x| :x} * foo", "char": 21},
-    { "src":  ".match {|x| :x} * {{foo}} extra", "char": 31},
-    { "src":  ".match |x| * {{foo}}", "char": 7},
-    { "src": ".match {|foo| :string} o:ne {{one}}  * {{other}}", "char": 24, "comment" : "tests for ':' in unquoted literals (not allowed)" },
-    { "src": ".match {|foo| :string} one: {{one}}  * {{other}}", "char": 26, "comment" : "tests for ':' in unquoted literals (not allowed)" },
-    { "src": ".local $foo = {|42| :number option=a:b} {{bar {$foo}}}", "char": 36, "comment" : "tests for ':' in unquoted literals (not allowed)" },
-    { "src": ".local $foo = {|42| :number option=a:b:c} {{bar {$foo}}}", "char": 36, "comment" : "tests for ':' in unquoted literals (not allowed)" },
-    { "src": "{$bar:foo}", "char": 5, "comment" : "tests for ':' in unquoted literals (not allowed)" },
-    {
-        "src":  ".match {1} {{_}}",
-        "char": 12,
-        "comment": "Disambiguating a wrong .match from an unsupported statement"
-    }
-]
diff --git a/icu4j/main/common_tests/src/test/java/com/ibm/icu/dev/test/message2/Mf2FeaturesTest.java b/icu4j/main/common_tests/src/test/java/com/ibm/icu/dev/test/message2/Mf2FeaturesTest.java
index 4bd787416b27..77c0fa3604d7 100644
--- a/icu4j/main/common_tests/src/test/java/com/ibm/icu/dev/test/message2/Mf2FeaturesTest.java
+++ b/icu4j/main/common_tests/src/test/java/com/ibm/icu/dev/test/message2/Mf2FeaturesTest.java
@@ -330,7 +330,7 @@ public void testSpecialPluralWithDecimals() {
         message = ".local $amount = {$count :number}\n"
                 + ".match {$amount :number}\n"
                 + "  1 {{I have {$amount} dollar.}}\n"
-                + "  * {{I have {$amount} dollars.}}\n";
+                + "  * {{I have {$amount} dollars.}}";
         TestUtils.runTestCase(new TestCase.Builder()
                 .pattern(message)
                 .locale("en-US")
@@ -340,7 +340,7 @@ public void testSpecialPluralWithDecimals() {
         message = ".local $amount = {$count :number minimumFractionDigits=2}\n"
                 + ".match {$amount :number minimumFractionDigits=2}\n"
                 + "  one {{I have {$amount} dollar.}}\n"
-                + "  *   {{I have {$amount} dollars.}}\n";
+                + "  *   {{I have {$amount} dollars.}}";
         TestUtils.runTestCase(new TestCase.Builder()
                 .pattern(message)
                 .locale("en-US")
@@ -369,7 +369,7 @@ public void testDefaultFunctionAndOptions() {
     public void testSimpleSelection() {
         String message = ".match {$count :number}\n"
                 + " 1 {{You have one notification.}}\n"
-                + " * {{You have {$count} notifications.}}\n";
+                + " * {{You have {$count} notifications.}}";
 
         TestUtils.runTestCase(new TestCase.Builder()
                 .pattern(message)
@@ -440,7 +440,7 @@ public void testLocaleVariableWithSelect() {
                 + ".local $exp = {$expDate :date year=numeric month=short day=numeric weekday=short}\n"
                 + ".match {$count :number}\n"
                 + " 1 {{Your ticket expires on {$exp}.}}\n"
-                + " * {{Your {$count} tickets expire on {$exp}.}}\n";
+                + " * {{Your {$count} tickets expire on {$exp}.}}";
 
         TestUtils.runTestCase(new TestCase.Builder()
                 .pattern(message)
diff --git a/icu4j/main/core/src/main/java/com/ibm/icu/message2/MFParser.java b/icu4j/main/core/src/main/java/com/ibm/icu/message2/MFParser.java
index 4a88bd04e1d6..073aafd8ef32 100644
--- a/icu4j/main/core/src/main/java/com/ibm/icu/message2/MFParser.java
+++ b/icu4j/main/core/src/main/java/com/ibm/icu/message2/MFParser.java
@@ -65,7 +65,6 @@ private MFDataModel.Message parseImpl() throws MFParseException {
             MFDataModel.Pattern pattern = getPattern();
             result = new MFDataModel.PatternMessage(new ArrayList<>(), pattern);
         }
-        skipOptionalWhitespaces();
         checkCondition(input.atEnd(), "Content detected after the end of the message.");
         new MFDataModelValidator(result).validate();
         return result;
@@ -174,14 +173,30 @@ private MFDataModel.Expression getPlaceholder() throws MFParseException {
         return result;
     }
 
-    private MFDataModel.Annotation getAnnotation() throws MFParseException {
+    private MFDataModel.Annotation getAnnotation(boolean whitespaceRequired) throws MFParseException {
         int position = input.getPosition();
-        skipOptionalWhitespaces();
 
+        // Handle absent annotation first (before parsing mandatory whitespace)
         int cp = input.peekChar();
+        if (cp == '}') {
+            return null;
+        }
+
+        int whitespaceCount = 0;
+        if (whitespaceRequired) {
+            whitespaceCount = skipMandatoryWhitespaces();
+        } else {
+            whitespaceCount = skipOptionalWhitespaces();
+        }
+
+        cp = input.peekChar();
         switch (cp) {
-            case '}':
+            case '}': {
+                // No annotation -- push the whitespace back,
+                // in case it's the required whitespace before an attribute
+                input.backup(whitespaceCount);
                 return null;
+            }
             case ':': // annotation, function
                 // abnf: function = ":" identifier *(s option)
                 input.readCodePoint(); // Consume the sigil
@@ -232,13 +247,13 @@ private MFDataModel.Expression getLiteralExpression() throws MFParseException {
         checkCondition(literal != null, "Literal expression expected.");
 
         MFDataModel.Annotation annotation = null;
-        int wsCount = skipWhitespaces();
-        if (wsCount > 0) { // we might have an annotation
-            annotation = getAnnotation();
+        boolean hasWhitespace = StringUtils.isWhitespace(input.peekChar());
+        if (hasWhitespace) { // we might have an annotation
+            annotation = getAnnotation(true);
             if (annotation == null) {
                 // We had some spaces, but no annotation.
                 // So we put (some) back for the possible attributes.
-                input.backup(1);
+             //   input.backup(1);
             }
         }
 
@@ -251,7 +266,7 @@ private MFDataModel.Expression getLiteralExpression() throws MFParseException {
     // abnf: variable-expression = "{" [s] variable [s annotation] *(s attribute) [s] "}"
     private MFDataModel.VariableExpression getVariableExpression() throws MFParseException {
         MFDataModel.VariableRef variableRef = getVariableRef();
-        MFDataModel.Annotation annotation = getAnnotation();
+        MFDataModel.Annotation annotation = getAnnotation(true);
         List<MFDataModel.Attribute> attributes = getAttributes();
         // Variable without a function, for example {$foo}
         return new MFDataModel.VariableExpression(variableRef, annotation, attributes);
@@ -259,7 +274,7 @@ private MFDataModel.VariableExpression getVariableExpression() throws MFParseExc
 
     // abnf: annotation-expression = "{" [s] annotation *(s attribute) [s] "}"
     private MFDataModel.Expression getAnnotationExpression() throws MFParseException {
-        MFDataModel.Annotation annotation = getAnnotation();
+        MFDataModel.Annotation annotation = getAnnotation(false);
         List<MFDataModel.Attribute> attributes = getAttributes();
 
         if (annotation instanceof MFDataModel.FunctionAnnotation) {
@@ -286,6 +301,9 @@ private MFDataModel.Markup getMarkup() throws MFParseException {
         MFDataModel.Annotation annotation = getMarkupAnnotation();
         List<MFDataModel.Attribute> attributes = getAttributes();
 
+        // Parse optional whitespace after attribute list
+        skipOptionalWhitespaces();
+
         cp = input.peekChar();
         if (cp == '/') {
             kind = MFDataModel.Markup.Kind.STANDALONE;
@@ -397,16 +415,28 @@ private String getIdentifier() throws MFParseException {
     // abnf helper: *(s option)
     private Map<String, MFDataModel.Option> getOptions() throws MFParseException {
         Map<String, MFDataModel.Option> options = new LinkedHashMap<>();
+        boolean first = true;
+        int skipCount = 0;
         while (true) {
             MFDataModel.Option option = getOption();
             if (option == null) {
                 break;
             }
+            // function = ":" identifier *(s option)
+            checkCondition(first || skipCount != 0,
+                           "Expected whitespace before option " + option.name);
+            first = false;
             if (options.containsKey(option.name)) {
                 error("Duplicated option '" + option.name + "'");
             }
             options.put(option.name, option);
+            // Can't just call skipMandatoryWhitespaces() here, because it
+            // might be the last option. So check for whitespace when
+            // parsing the next option instead.
+            skipCount = skipOptionalWhitespaces();
         }
+        // Restore the last chunk of whitespace in case there's an attribute following
+        input.backup(skipCount);
         return options;
     }
 
@@ -422,7 +452,7 @@ private MFDataModel.Option getOption() throws MFParseException {
         skipOptionalWhitespaces();
         int cp = input.readCodePoint();
         checkCondition(cp == '=', "Expected '='");
-        // skipOptionalWhitespaces();
+        skipOptionalWhitespaces();
         MFDataModel.LiteralOrVariableRef litOrVar = getLiteralOrVariableRef();
         if (litOrVar == null) {
             error("Options must have a value. An empty string should be quoted.");
@@ -478,8 +508,9 @@ private MFDataModel.Literal getQuotedLiteral() throws MFParseException {
                 result.appendCodePoint(cp);
             } else if (cp == '\\') {
                 cp = input.readCodePoint();
-                checkCondition(cp == '|', "Invalid escape sequence, only \"\\|\" is valid here");
-                result.appendCodePoint('|');
+                boolean isValidEscape = cp == '|' || cp == '\\' || cp == '{' || cp == '}';
+                checkCondition(isValidEscape, "Invalid escape sequence inside quoted literal");
+                result.appendCodePoint(cp);
             } else {
                 break;
             }
@@ -511,13 +542,14 @@ private MFDataModel.Literal getNumberLiteral() {
         return null;
     }
 
-    private void skipMandatoryWhitespaces() throws MFParseException {
+    private int skipMandatoryWhitespaces() throws MFParseException {
         int count = skipWhitespaces();
         checkCondition(count > 0, "Space expected");
+        return count;
     }
 
-    private void skipOptionalWhitespaces() {
-        skipWhitespaces();
+    private int skipOptionalWhitespaces() {
+        return skipWhitespaces();
     }
 
     private int skipWhitespaces() {
@@ -554,13 +586,11 @@ private MFDataModel.Message getComplexMessage() throws MFParseException {
         } else { // Expect {{...}} or end of message
             skipOptionalWhitespaces();
             int cp = input.peekChar();
-            if (cp == EOF) {
-                // Only declarations, no pattern
-                return new MFDataModel.PatternMessage(declarations, null);
-            } else {
-                MFDataModel.Pattern pattern = getQuotedPattern();
-                return new MFDataModel.PatternMessage(declarations, pattern);
-            }
+            // complex-message   = *(declaration [s]) complex-body
+            checkCondition(cp != EOF, "Expected a quoted pattern or .match; got end-of-input");
+            MFDataModel.Pattern pattern = getQuotedPattern();
+            checkCondition(input.atEnd(), "Content detected after the end of the message.");
+            return new MFDataModel.PatternMessage(declarations, pattern);
         }
     }
 
@@ -576,7 +606,11 @@ private MFDataModel.SelectMessage getMatch(List<MFDataModel.Declaration> declara
         // Look for selectors
         List<MFDataModel.Expression> expressions = new ArrayList<>();
         while (true) {
-            skipMandatoryWhitespaces();
+            // Whitespace not required between selectors:
+            // match 1*([s] selector)
+            // Whitespace not required before first variant:
+            // matcher = match-statement 1*([s] variant)
+            skipOptionalWhitespaces();
             MFDataModel.Expression expression = getPlaceholder();
             if (expression == null) {
                 break;
@@ -597,6 +631,7 @@ private MFDataModel.SelectMessage getMatch(List<MFDataModel.Declaration> declara
             }
             variants.add(variant);
         }
+        checkCondition(input.atEnd(), "Content detected after the end of the message.");
         return new MFDataModel.SelectMessage(declarations, expressions, variants);
     }
 
@@ -613,7 +648,12 @@ private MFDataModel.Variant getVariant() throws MFParseException {
             }
             keys.add(key);
         }
-        skipOptionalWhitespaces();
+        // Only want to skip whitespace if we parsed at least one key --
+        // otherwise, we might fail to catch trailing whitespace at the end of
+        // the message, which is a parse error
+        if (!keys.isEmpty()) {
+            skipOptionalWhitespaces();
+        }
         if (input.atEnd()) {
             checkCondition(
                     keys.isEmpty(), "After selector keys it is mandatory to have a pattern.");
@@ -624,17 +664,26 @@ private MFDataModel.Variant getVariant() throws MFParseException {
     }
 
     private MFDataModel.LiteralOrCatchallKey getKey(boolean requireSpaces) throws MFParseException {
+        int cp = input.peekChar();
+        // Whitespace not required between last key and pattern:
+        // variant = key *(s key) [s] quoted-pattern
+        if (cp == '{') {
+            return null;
+        }
+        int skipCount = 0;
         if (requireSpaces) {
-            skipMandatoryWhitespaces();
+            skipCount = skipMandatoryWhitespaces();
         } else {
-            skipOptionalWhitespaces();
+            skipCount = skipOptionalWhitespaces();
         }
-        int cp = input.peekChar();
+        cp = input.peekChar();
         if (cp == '*') {
             input.readCodePoint(); // consume the '*'
             return new MFDataModel.CatchallKey();
         }
         if (cp == EOF) {
+            // Restore whitespace, in order to detect the error case of whitespace at the end of a message
+            input.backup(skipCount);
             return null;
         }
         return getLiteral();
@@ -666,14 +715,11 @@ private MFDataModel.Declaration getDeclaration() throws MFParseException {
                 skipMandatoryWhitespaces();
                 expression = getPlaceholder();
                 String inputVarName = null;
-                if (expression instanceof MFDataModel.VariableExpression) {
-                    inputVarName = ((MFDataModel.VariableExpression) expression).arg.name;
-                }
-                if (expression instanceof MFDataModel.VariableExpression) {
-                    return new MFDataModel.InputDeclaration(
-                            inputVarName, (MFDataModel.VariableExpression) expression);
-                }
-                break;
+                checkCondition(expression instanceof MFDataModel.VariableExpression,
+                               "Variable expression required in .input declaration");
+                inputVarName = ((MFDataModel.VariableExpression) expression).arg.name;
+                return new MFDataModel.InputDeclaration(inputVarName,
+                                                        (MFDataModel.VariableExpression) expression);
             case "local":
                 // abnf: local-declaration = local s variable [s] "=" [s] expression
                 skipMandatoryWhitespaces();
@@ -696,6 +742,17 @@ private MFDataModel.Declaration getDeclaration() throws MFParseException {
                 List<MFDataModel.Expression> expressions = new ArrayList<>();
                 while (true) {
                     skipOptionalWhitespaces();
+                    // Look ahead to detect the end of the unsupported statement
+                    // (next token either begins a placeholder or begins a complex body)
+                    cp = input.readCodePoint();
+                    int cp1 = input.readCodePoint();
+                    if (cp == '{' && cp1 == '{') {
+                        // End of unsupported statement
+                        input.backup(2);
+                        break;
+                    } else {
+                        input.backup(2);
+                    }
                     expression = getPlaceholder();
                     // This also covers != null
                     if (expression instanceof MFDataModel.VariableExpression) {
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CoreTest.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CoreTest.java
index 66c40ccfbdc9..3305f2435a50 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CoreTest.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CoreTest.java
@@ -14,14 +14,27 @@
 @SuppressWarnings({"static-method", "javadoc"})
 @RunWith(JUnit4.class)
 public class CoreTest extends CoreTestFmwk {
-    private static final String JSON_FILE = "test-core.json";
+    private static final String[] JSON_FILES = {"alias-selector-annotations.json",
+                                                "duplicate-declarations.json",
+                                                "invalid-options.json",
+                                                "markup.json",
+                                                "matches-whitespace.json",
+                                                "reserved-syntax.json",
+                                                "resolution-errors.json",
+                                                "runtime-errors.json",
+                                                "spec/test-core.json",
+                                                "syntax-errors-diagnostics.json",
+                                                "tricky-declarations.json",
+                                                "valid-tests.json"};
 
     @Test
     public void test() throws Exception {
-        try (Reader reader = TestUtils.jsonReader(JSON_FILE)) {
-            Unit[] unitList = TestUtils.GSON.fromJson(reader, Unit[].class);
-            for (Unit unit : unitList) {
-                TestUtils.runTestCase(unit);
+        for (String jsonFile : JSON_FILES) {
+            try (Reader reader = TestUtils.jsonReader(jsonFile)) {
+                Unit[] unitList = TestUtils.GSON.fromJson(reader, Unit[].class);
+                for (Unit unit : unitList) {
+                    TestUtils.runTestCase(unit);
+                }
             }
         }
     }
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CustomFormatterPersonTest.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CustomFormatterPersonTest.java
index 0fec7a46b4e5..97dcfb3f287d 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CustomFormatterPersonTest.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CustomFormatterPersonTest.java
@@ -165,7 +165,7 @@ public void testCustomFunctionsComplexMessage() {
                 + " * 0 {{{$hostName} does not give a party.}}\n"
                 + " * 1 {{{$hostName} invites {$guestName} to their party.}}\n"
                 + " * 2 {{{$hostName} invites {$guestName} and one other person to their party.}}\n"
-                + " * * {{{$hostName} invites {$guestName} and {$guestsOther} other people to their party.}}\n";
+                + " * * {{{$hostName} invites {$guestName} and {$guestsOther} other people to their party.}}";
 
         TestUtils.runTestCase(CUSTOM_FUNCTION_REGISTRY, new TestCase.Builder()
                 .pattern(message)
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/DataModelErrorsTest.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/DataModelErrorsTest.java
index 162fade64c92..d8ba5699c260 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/DataModelErrorsTest.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/DataModelErrorsTest.java
@@ -19,20 +19,23 @@
 @SuppressWarnings({"static-method", "javadoc"})
 @RunWith(JUnit4.class)
 public class DataModelErrorsTest extends CoreTestFmwk {
-    private static final String JSON_FILE = "data-model-errors.json";
+    private static final String[] JSON_FILES = {"spec/data-model-errors.json",
+                                                "more-data-model-errors.json"};
 
     @Test
     public void test() throws Exception {
-        try (Reader reader = TestUtils.jsonReader(JSON_FILE)) {
-            Type mapType = new TypeToken<Map<String, String[]>>(){/* not code */}.getType();
-            Map<String, String[]> unitList = TestUtils.GSON.fromJson(reader, mapType);
-            for (Entry<String, String[]> tests : unitList.entrySet()) {
-                for (String pattern : tests.getValue()) {
-                    try {
-                        MessageFormatter.builder().setPattern(pattern).build().formatToString(null);
-                        fail("Undetected errors in '" + tests.getKey() + "': '" + pattern + "'");
-                    } catch (Exception e) {
-                        // We expected an error, so it's all good
+        for (String jsonFile : JSON_FILES) {
+            try (Reader reader = TestUtils.jsonReader(jsonFile)) {
+                Type mapType = new TypeToken<Map<String, String[]>>(){/* not code */}.getType();
+                Map<String, String[]> unitList = TestUtils.GSON.fromJson(reader, mapType);
+                for (Entry<String, String[]> tests : unitList.entrySet()) {
+                    for (String pattern : tests.getValue()) {
+                        try {
+                            MessageFormatter.builder().setPattern(pattern).build().formatToString(null);
+                            fail("Undetected errors in '" + tests.getKey() + "': '" + pattern + "'");
+                        } catch (Exception e) {
+                            // We expected an error, so it's all good
+                        }
                     }
                 }
             }
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/FunctionsTest.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/FunctionsTest.java
index 6a3e15df0a09..c76c60a50327 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/FunctionsTest.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/FunctionsTest.java
@@ -18,16 +18,19 @@
 @SuppressWarnings({"static-method", "javadoc"})
 @RunWith(JUnit4.class)
 public class FunctionsTest extends CoreTestFmwk {
-    private static final String JSON_FILE = "test-functions.json";
+    private static final String[] JSON_FILES = {"spec/test-functions.json",
+                                                "more-functions.json"};
 
     @Test
     public void test() throws Exception {
-        try (Reader reader = TestUtils.jsonReader(JSON_FILE)) {
-            Type mapType = new TypeToken<Map<String, Unit[]>>(){/* not code */}.getType();
-            Map<String, Unit[]> unitList = TestUtils.GSON.fromJson(reader, mapType);
-            for (Entry<String, Unit[]> testGroup : unitList.entrySet()) {
-                for (Unit unit : testGroup.getValue()) {
-                    TestUtils.runTestCase(unit);
+        for (String jsonFile : JSON_FILES) {
+            try (Reader reader = TestUtils.jsonReader(jsonFile)) {
+                Type mapType = new TypeToken<Map<String, Unit[]>>(){/* not code */}.getType();
+                Map<String, Unit[]> unitList = TestUtils.GSON.fromJson(reader, mapType);
+                for (Entry<String, Unit[]> testGroup : unitList.entrySet()) {
+                    for (Unit unit : testGroup.getValue()) {
+                        TestUtils.runTestCase(unit);
+                    }
                 }
             }
         }
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/IcuFunctionsTest.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/IcuFunctionsTest.java
index 7930e6ccb18f..bd0af39d085c 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/IcuFunctionsTest.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/IcuFunctionsTest.java
@@ -21,13 +21,6 @@
 public class IcuFunctionsTest extends CoreTestFmwk {
     private static final String JSON_FILE = "icu-test-functions.json";
 
-    // Some default parameters for all messages, to use if the message does not have its own
-    private static final Map<String, Object> ARGS =
-            Args.of(
-                    "user", "John",
-                    "exp", new Date(2024 - 1900, 7, 3, 21, 43, 57), // Aug 3, 2024, at 9:43:57 pm
-                    "tsOver", "full");
-
     @Test
     public void test() throws Exception {
         try (Reader reader = TestUtils.jsonReader(JSON_FILE)) {
@@ -38,7 +31,7 @@ public void test() throws Exception {
             Map<String, Unit[]> unitList = TestUtils.GSON.fromJson(reader, mapType);
             for (Entry<String, Unit[]> testGroup : unitList.entrySet()) {
                 for (Unit unit : testGroup.getValue()) {
-                    TestUtils.runTestCase(unit, ARGS);
+                    TestUtils.runTestCase(unit);
                 }
             }
         }
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/MessageFormat2Test.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/MessageFormat2Test.java
index 33005b33c893..6ac37d6085fd 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/MessageFormat2Test.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/MessageFormat2Test.java
@@ -131,7 +131,7 @@ public void testPlural() {
         String message = ""
                 + ".match {$count :number}\n"
                 + " 1 {{You have one notification.}}\n"
-                + " * {{You have {$count} notifications.}}\n";
+                + " * {{You have {$count} notifications.}}";
 
         MessageFormatter mf2 = MessageFormatter.builder()
                 .setPattern(message)
@@ -154,7 +154,7 @@ public void testPluralOrdinal() {
                 + " one {{You got in the {$place}st place}}\n"
                 + " two {{You got in the {$place}nd place}}\n"
                 + " few {{You got in the {$place}rd place}}\n"
-                + "  *  {{You got in the {$place}th place}}\n"
+                + "  *  {{You got in the {$place}th place}}"
                 ;
 
         MessageFormatter mf2 = MessageFormatter.builder()
@@ -336,7 +336,7 @@ public void testPluralWithOffset() {
                 + " 1   {{Anna}}\n"
                 + " 2   {{Anna and Bob}}\n"
                 + " one {{Anna, Bob, and {$count :number icu:offset=2} other guest}}\n"
-                + " *   {{Anna, Bob, and {$count :number icu:offset=2} other guests}}\n";
+                + " *   {{Anna, Bob, and {$count :number icu:offset=2} other guests}}";
         MessageFormatter mf2 = MessageFormatter.builder()
                 .setPattern(message)
                 .build();
@@ -365,7 +365,7 @@ public void testPluralWithOffsetAndLocalVar() {
                 + " 1   {{Anna}}\n"
                 + " 2   {{Anna and Bob}}\n"
                 + " one {{Anna, Bob, and {$foo} other guest}}\n"
-                + " *   {{Anna, Bob, and {$foo} other guests}}\n";
+                + " *   {{Anna, Bob, and {$foo} other guests}}";
         MessageFormatter mf2 = MessageFormatter.builder()
                 .setPattern(message)
                 .build();
@@ -393,7 +393,7 @@ public void testPluralWithOffsetAndLocalVar2() {
                 + ".match {$foo :number}\n" // should "inherit" the offset
                 + " 1   {{Last dollar}}\n"
                 + " one {{{$foo} dollar}}\n"
-                + " *   {{{$foo} dollars}}\n";
+                + " *   {{{$foo} dollars}}";
         MessageFormatter mf2 = MessageFormatter.builder()
                 .setPattern(message)
                 .build();
@@ -415,7 +415,7 @@ public void testPluralWithOffsetAndLocalVar2Options() {
                 + ".match {$foo :number}\n" // should "inherit" the offset
                 + " 1   {{Last dollar}}\n"
                 + " one {{{$foo} dollar}}\n"
-                + " *   {{{$foo} dollars}}\n";
+                + " *   {{{$foo} dollars}}";
         MessageFormatter mf2 = MessageFormatter.builder()
                 .setPattern(message)
                 .build();
@@ -436,7 +436,7 @@ public void testLoopOnLocalVars() {
                 + ".local $foo = {$baz :number}\n"
                 + ".local $bar = {$foo}\n"
                 + ".local $baz = {$bar}\n"
-                + "{{The message uses {$baz} and works}}\n";
+                + "{{The message uses {$baz} and works}}";
         // Circular references on variables is now detected.
         // So we check that this throws (see expected in the @Test above)
         MessageFormatter.builder()
@@ -451,7 +451,7 @@ public void testVariableOptionsInSelector() {
                 + " 1   {{A}}\n"
                 + " 2   {{A and B}}\n"
                 + " one {{A, B, and {$count :number icu:offset=$delta} more character}}\n"
-                + " *   {{A, B, and {$count :number icu:offset=$delta} more characters}}\n";
+                + " *   {{A, B, and {$count :number icu:offset=$delta} more characters}}";
         MessageFormatter mfVar = MessageFormatter.builder()
                 .setPattern(messageVar)
                 .build();
@@ -468,7 +468,7 @@ public void testVariableOptionsInSelector() {
                 + ".match {$count :number icu:offset=$delta}\n"
                 + " 1 {{Exactly 1}}\n"
                 + " 2 {{Exactly 2}}\n"
-                + " * {{Count = {$count :number icu:offset=$delta} and delta={$delta}.}}\n";
+                + " * {{Count = {$count :number icu:offset=$delta} and delta={$delta}.}}";
         MessageFormatter mfVar2 = MessageFormatter.builder()
                 .setPattern(messageVar2)
                 .build();
@@ -509,7 +509,7 @@ public void testVariableOptionsInSelectorWithLocalVar() {
                 + " 1   {{A}}\n"
                 + " 2   {{A and B}}\n"
                 + " one {{A, B, and {$offCount} more character}}\n"
-                + " *   {{A, B, and {$offCount} more characters}}\n";
+                + " *   {{A, B, and {$offCount} more characters}}";
         MessageFormatter mfFix = MessageFormatter.builder()
                 .setPattern(messageFix)
                 .build();
@@ -524,7 +524,7 @@ public void testVariableOptionsInSelectorWithLocalVar() {
                 + " 1   {{A}}\n"
                 + " 2   {{A and B}}\n"
                 + " one {{A, B, and {$offCount} more character}}\n"
-                + " *   {{A, B, and {$offCount} more characters}}\n";
+                + " *   {{A, B, and {$offCount} more characters}}";
         MessageFormatter mfVar = MessageFormatter.builder()
                 .setPattern(messageVar)
                 .build();
@@ -542,7 +542,7 @@ public void testVariableOptionsInSelectorWithLocalVar() {
                 + ".match {$offCount :number}\n"
                 + " 1 {{Exactly 1}}\n"
                 + " 2 {{Exactly 2}}\n"
-                + " * {{Count = {$count}, OffCount = {$offCount}, and delta={$delta}.}}\n";
+                + " * {{Count = {$count}, OffCount = {$offCount}, and delta={$delta}.}}";
         MessageFormatter mfVar2 = MessageFormatter.builder()
                 .setPattern(messageVar2)
                 .build();
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/SerializationTest.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/SerializationTest.java
index 01e18c586df4..2655d42569fe 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/SerializationTest.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/SerializationTest.java
@@ -43,12 +43,12 @@ public void test() throws Exception {
                     + "1   {{You got the gold medal}}\n"
                     + "2   {{You got the silver medal}}\n"
                     + "3   {{You got the bronze medal}}\n"
-                    + "few {{You fininshed in the {$place}rd place}}\n",
+                    + "few {{You fininshed in the {$place}rd place}}",
             ".match {$fileCount :number} {$folderCount :number}\n"
                     + "*   *   {{You deleted {$fileCount} files in {$folderCount} folders}}\n"
                     + "one one {{You deleted {$fileCount} file in {$folderCount} folder}}\n"
                     + "one *   {{You deleted {$fileCount} file in {$folderCount} folders}}\n"
-                    + "*   one {{You deleted {$fileCount} files in {$folderCount} folder}}\n",
+                    + "*   one {{You deleted {$fileCount} files in {$folderCount} folder}}",
             "{$count :number minimumFractionDigits=2} dollars",
             "{$count :number minimumFractionDigits=3} dollars",
             "{|3.1415| :number minimumFractionDigits=5} dollars",
@@ -57,7 +57,7 @@ public void test() throws Exception {
                     + ".match {$c}\n"
                     + "one {{{$c} dollar}}\n"
                     + "*   {{{$c} dollars}}",
-            ".local $c = {$count} .foobar |asd asd asd asd| {$bar1} {$bar2} {$bar3} .local $b = {$bar} {{Foo bar}}\n",
+            ".local $c = {$count} .foobar |asd asd asd asd| {$bar1} {$bar2} {$bar3} .local $b = {$bar} {{Foo bar}}",
             ".local $c = {1 :number minimumFractionDigits=2}\n"
                     + ".match {$c}\n"
                     + "one {{{$c} dollar}}\n"
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/SyntaxErrorsTest.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/SyntaxErrorsTest.java
index 30ad47f189a2..204f79555161 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/SyntaxErrorsTest.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/SyntaxErrorsTest.java
@@ -15,18 +15,21 @@
 @SuppressWarnings({"static-method", "javadoc"})
 @RunWith(JUnit4.class)
 public class SyntaxErrorsTest extends CoreTestFmwk {
-    private static final String JSON_FILE = "syntax-errors.json";
+    private static final String[] JSON_FILES = {"more-syntax-errors.json",
+                                                "spec/syntax-errors.json"};
 
     @Test
     public void test() throws Exception {
-        try (Reader reader = TestUtils.jsonReader(JSON_FILE)) {
-            String[] srcList = TestUtils.GSON.fromJson(reader, String[].class);
-            for (String source : srcList) {
-                try {
-                    MessageFormatter.builder().setPattern(source).build();
-                    fail("Pattern expected to fail, but didn't: '" + source + "'");
-                } catch (Exception e) {
-                    // If we get here it is fine
+        for (String jsonFile : JSON_FILES) {
+            try (Reader reader = TestUtils.jsonReader(jsonFile)) {
+                String[] srcList = TestUtils.GSON.fromJson(reader, String[].class);
+                for (String source : srcList) {
+                    try {
+                        MessageFormatter.builder().setPattern(source).build();
+                        fail("Pattern expected to fail, but didn't: '" + source + "'");
+                    } catch (Exception e) {
+                        // If we get here it is fine
+                    }
                 }
             }
         }
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/TestUtils.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/TestUtils.java
index ae8868d03f20..572da833e67a 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/TestUtils.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/TestUtils.java
@@ -14,6 +14,7 @@
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.util.Date;
 import java.util.Locale;
 import java.util.Map;
 
@@ -71,6 +72,42 @@ private static String reportCase(TestCase testCase) {
 
     // ======= Same functionality with Unit, usable with JSON ========
 
+    static void rewriteDates(Map<String, Object> params) {
+        // For each value in `params` that's a map with the single key
+        // `date` and a double value d,
+        // return a map with that value changed to Date(d)
+        // In JSON this looks like:
+        //    "params": {"exp": { "date": 1722746637000 } }
+        for (Map.Entry<String, Object> pair : params.entrySet()) {
+            if (pair.getValue() instanceof Map) {
+                Map innerMap = (Map) pair.getValue();
+                if (innerMap.size() == 1 && innerMap.containsKey("date") && innerMap.get("date") instanceof Double) {
+                    Long dateValue = Double.valueOf((Double) innerMap.get("date")).longValue();
+                    params.put(pair.getKey(), new Date(dateValue));
+                }
+            }
+        }
+    }
+
+    static void rewriteDecimals(Map<String, Object> params) {
+        // For each value in `params` that's a map with the single key
+        // `decimal` and a string value s
+        // return a map with that value changed to Decimal(s)
+        // In JSON this looks like:
+        //    "params": {"val": {"decimal": "1234567890123456789.987654321"}},
+        for (Map.Entry<String, Object> pair : params.entrySet()) {
+            if (pair.getValue() instanceof Map) {
+                Map innerMap = (Map) pair.getValue();
+                if (innerMap.size() == 1 && innerMap.containsKey("decimal")
+                    && innerMap.get("decimal") instanceof String) {
+                    String decimalValue = (String) innerMap.get("decimal");
+                    params.put(pair.getKey(), new com.ibm.icu.math.BigDecimal(decimalValue));
+                }
+            }
+        }
+    }
+
+
     static boolean expectsErrors(Unit unit) {
         return unit.errors != null && !unit.errors.isEmpty();
     }
@@ -80,7 +117,7 @@ static void runTestCase(Unit unit) {
     }
 
     static void runTestCase(Unit unit, Map<String, Object> params) {
-        if (unit.ignore != null) {
+        if (unit.ignoreJava != null) {
             return;
         }
 
@@ -107,6 +144,8 @@ static void runTestCase(Unit unit, Map<String, Object> params) {
             MessageFormatter mf = mfBuilder.build();
             if (unit.params != null) {
                 params = unit.params;
+                rewriteDates(params);
+                rewriteDecimals(params);
             }
             String result = mf.formatToString(params);
             if (expectsErrors(unit)) {
@@ -134,10 +173,25 @@ static Reader jsonReader(String jsonFileName) throws URISyntaxException, IOExcep
         return Files.newBufferedReader(json, StandardCharsets.UTF_8);
     }
 
-    private static Path getTestFile(Class<?> cls, String fileName) throws URISyntaxException {
+    private static Path getTestFile(Class<?> cls, String fileName) throws URISyntaxException, IOException {
         String packageName = cls.getPackage().getName().replace('.', '/');
         URI getPath = cls.getClassLoader().getResource(packageName).toURI();
         Path filePath = Paths.get(getPath);
         Path json = Paths.get(fileName);
-        return filePath.resolve(json);
+        // First, check the top level of the source directory,
+        // in case we're in a source tarball
+        Path icuTestdataInSourceDir = filePath.resolve("../../../../../../../../../../../testdata/message2/").normalize();
+        Path icuTestdataDir = icuTestdataInSourceDir;
+        if (!Files.isDirectory(icuTestdataInSourceDir)) {
+            // If that doesn't exist, check one directory higher, in case we're
+            // in a checked-out repo
+            Path icuTestdataInRepo = Paths.get("../").resolve(icuTestdataInSourceDir).normalize();
+            if (!Files.isDirectory(icuTestdataInRepo)) {
+                throw new java.io.FileNotFoundException("Test data directory does not exist: tried "
+                                                        + icuTestdataInSourceDir + " and "
+                                                        + icuTestdataInRepo);
+            }
+            icuTestdataDir = icuTestdataInSourceDir;
+        }
+        return icuTestdataDir.resolve(json);
     }}
diff --git a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/Unit.java b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/Unit.java
index facacca75234..3ff18ecd7cc2 100644
--- a/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/Unit.java
+++ b/icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/Unit.java
@@ -15,7 +15,7 @@ class Unit {
     final String locale;
     final Map<String, Object> params;
     final String exp;
-    final String ignore;
+    final String ignoreJava;
     final List<Error> errors;
 
     Unit(
@@ -24,14 +24,14 @@ class Unit {
             String locale,
             Map<String, Object> params,
             String exp,
-            String ignore,
+            String ignoreJava,
             List<Error> errors) {
         this.src = src;
         this.srcs = srcs;
         this.locale = locale;
         this.params = params;
         this.exp = exp;
-        this.ignore = ignore;
+        this.ignoreJava = ignoreJava;
         this.errors = errors;
     }
 
@@ -80,7 +80,7 @@ public Unit merge(Unit other) {
         String newLocale = other.locale != null ? other.locale : this.locale;
         Map<String, Object> newParams = other.params != null ? other.params : this.params;
         String newExp = other.exp != null ? other.exp : this.exp;
-        String newIgnore = other.ignore != null ? other.ignore : this.ignore;
+        String newIgnore = other.ignoreJava != null ? other.ignoreJava : this.ignoreJava;
         List<Error> newErrors = other.errors != null ? other.errors : this.errors;
         return new Unit(newSrc, newSrcs, newLocale, newParams, newExp, newIgnore, newErrors);
     }
diff --git a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/data-model-errors.json b/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/data-model-errors.json
deleted file mode 100644
index 0a6bd67641b6..000000000000
--- a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/data-model-errors.json
+++ /dev/null
@@ -1,32 +0,0 @@
-{
-    "Variant Key Mismatch": [
-        ".match {$foo :x} * * {{foo}}",
-        ".match {$foo :x} {$bar :x} * {{foo}}"
-    ],
-    "Missing Fallback Variant": [
-        ".match {:foo} 1 {{_}}",
-        ".match {:foo} other {{_}}",
-        ".match {:foo} {:bar} * 1 {{_}} 1 * {{_}}"
-    ],
-    "Missing Selector Annotation": [
-        ".match {$foo} one {{one}} * {{other}}",
-        ".input {$foo} .match {$foo} one {{one}} * {{other}}",
-        ".local $foo = {$bar} .match {$foo} one {{one}} * {{other}}"
-    ],
-    "Duplicate Declaration": [
-        ".input {$foo} .input {$foo} {{_}}",
-        ".input {$foo} .local $foo = {42} {{_}}",
-        ".local $foo = {42} .input {$foo} {{_}}",
-        ".local $foo = {:unknown} .local $foo = {42} {{_}}",
-        ".local $foo = {$bar} .local $bar = {42} {{_}}",
-        ".local $foo = {$foo} {{_}}",
-        ".local $foo = {$bar} .local $bar = {$baz} {{_}}",
-        ".local $foo = {$bar :func} .local $bar = {$baz} {{_}}",
-        ".local $foo = {42 :func opt=$foo} {{_}}",
-        ".local $foo = {42 :func opt=$bar} .local $bar = {42} {{_}}"
-    ],
-    "Duplicate Option Name": [
-            "bad {:placeholder option=x option=x}",
-            "bad {:placeholder ns:option=x ns:option=y}"
-    ]
-}
diff --git a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-parser-tests.json b/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-parser-tests.json
deleted file mode 100644
index 56e3094e58d7..000000000000
--- a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-parser-tests.json
+++ /dev/null
@@ -1,61 +0,0 @@
-{
-  "Simple messages": [
-    "",
-    "Hello",
-    "Hello world!",
-    "Hello \\t \\n \\r \\{ world!",
-    "Hello world {:datetime}",
-    "Hello world {foo}",
-    "Hello {0} world",
-    "Hello {123} world",
-    "Hello {-123} world",
-    "Hello {3.1416} world",
-    "Hello {-3.1416} world",
-    "Hello {123E+2} world",
-    "Hello {123E-2} world",
-    "Hello {123.456E+2} world",
-    "Hello {123.456E-2} world",
-    "Hello {-123.456E+2} world",
-    "Hello {-123.456E-2} world",
-    "Hello {-123E+2} world",
-    "Hello {-123E-2} world",
-    "Hello world {$exp}",
-    "Hello world {$exp :datetime}",
-    "Hello world {|2024-02-27| :datetime}",
-    "Hello world {$exp :datetime style=long} and more",
-    "Hello world {$exp :function number=1234} and more",
-    "Hello world {$exp :function unquoted=left   } and more",
-    "Hello world {$exp :function quoted=|Something| } and more",
-    "Hello world {$exp :function quoted=|Something with spaces| } and more",
-    "Hello world {$exp :function quoted=|Something with \\| spaces and \\| escapes| } and more",
-    "Hello world {$exp :function number=1234 unquoted=left quoted=|Something|}",
-    "Hello world {$exp :function number=1234 unquoted=left quoted=|Something longer|}",
-    "Hello world {$exp :function number=1234 unquoted=left quoted=|Something \\| longer|}"
-  ],
-  "Attributes": [
-    "Hello world {$exp}",
-    "Hello world {$exp @attr}",
-    "Hello world {$exp @valid @attr=a @attrb=123 @atrn=|foo bar|}",
-    "Hello world {$exp :date @valid @attr=aaaa @attrb=123 @atrn=|foo bar|}",
-    "Hello world {$exp :date year=numeric month=long day=numeric int=12 @valid @attr=a @attrb=123 @atrn=|foo bar|}"
-    ],
-  "Reserved and private": [
-    "Reserved {$exp &foo |something more protected|} and more",
-    "Reserved {$exp %foo |something quoted \\| inside|} and more",
-    "{{.starting with dot is OK here}}",
-    "{{Some string pattern, with {$foo} and {$exp :date style=long}!}}"
-  ],
-  "Simple messages, with declarations": [
-    ".input {$pi :number}",
-    ".input {$exp :date}",
-    ".local $foo = {$exp}",
-    ".local $foo = {$exp :date}",
-    ".local $foo = {$exp :date year=numeric month=long day=numeric}",
-    ".local $bar = {$foo :date month=medium}",
-    ".something |reserved=| {$foo :date}"
-  ],
-  "Multiple declarations in one message": [
-    ".input {$a :date} .local $b = {$a :date year=numeric month=long day=numeric} .local $c = {$b :date month=medium} .someting |reserved = \\| and more| {$x :date} {$y :date} {$z :number}",
-    ".input {$a :date} .local $exp = {$a :date style=full} {{Your card expires on {$exp}!}}"
-  ]
-}
diff --git a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/syntax-errors.json b/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/syntax-errors.json
deleted file mode 100644
index fc4537131c83..000000000000
--- a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/syntax-errors.json
+++ /dev/null
@@ -1,56 +0,0 @@
-[
-  ".",
-  "{",
-  "}",
-  "{}",
-  "{{",
-  "{{}",
-  "{{}}}",
-  "{|foo| #markup}",
-  "{{missing end brace}",
-  "{{missing end braces",
-  "{{missing end {$braces",
-  "{{extra}} content",
-  "empty { } placeholder",
-  "missing space {42:func}",
-  "missing space {|foo|:func}",
-  "missing space {|foo|@bar}",
-  "missing space {:func@bar}",
-  "missing space {:func @bar@baz}",
-  "missing space {:func @bar=42@baz}",
-  "missing space {+reserved@bar}",
-  "missing space {&private@bar}",
-  "bad {:} placeholder",
-  "bad {\\u0000placeholder}",
-  "no-equal {|42| :number minimumFractionDigits 2}",
-  "bad {:placeholder option=}",
-  "bad {:placeholder option value}",
-  "bad {:placeholder option:value}",
-  "bad {:placeholder option}",
-  "bad {:placeholder:}",
-  "bad {::placeholder}",
-  "bad {:placeholder::foo}",
-  "bad {:placeholder option:=x}",
-  "bad {:placeholder :option=x}",
-  "bad {:placeholder option::x=y}",
-  "bad {$placeholder option}",
-  "bad {:placeholder @attribute=}",
-  "bad {:placeholder @attribute=@foo}",
-  "no {placeholder end",
-  "no {$placeholder end",
-  "no {:placeholder end",
-  "no {|placeholder| end",
-  "no {|literal} end",
-  "no {|literal or placeholder end",
-  ".local bar = {|foo|} {{_}}",
-  ".local #bar = {|foo|} {{_}}",
-  ".local $bar {|foo|} {{_}}",
-  ".local $bar = |foo| {{_}}",
-  ".match {#foo} * {{foo}}",
-  ".match {} * {{foo}}",
-  ".match {|foo| :x} {|bar| :x} ** {{foo}}",
-  ".match * {{foo}}",
-  ".match {|x| :x} * foo",
-  ".match {|x| :x} * {{foo}} extra",
-  ".match |x| * {{foo}}"
-]
diff --git a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/test-core.json b/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/test-core.json
deleted file mode 100644
index 4400b69774ee..000000000000
--- a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/test-core.json
+++ /dev/null
@@ -1,212 +0,0 @@
-[
-  { "src": "hello", "exp": "hello" },
-  { "src": "hello {world}", "exp": "hello world" },
-  {
-    "src": "hello { world\t\n}",
-    "exp": "hello world",
-    "cleanSrc": "hello {world}"
-  },
-  {
-    "src": "hello {\u3000world\r}",
-    "exp": "hello world",
-    "cleanSrc": "hello {world}"
-  },
-  { "src": "hello {|world|}", "exp": "hello world" },
-  { "src": "hello {||}", "exp": "hello " },
-  {
-    "src": "hello {$place}",
-    "params": { "place": "world" },
-    "exp": "hello world"
-  },
-  {
-    "src": "hello {$place-.}",
-    "params": { "place-.": "world" },
-    "exp": "hello world"
-  },
-  {
-    "src": "hello {$place}",
-    "errorsJs": [{ "type": "unresolved-var" }],
-    "exp": "hello {$place}"
-  },
-  {
-    "src": "{$one} and {$two}",
-    "params": { "one": 1.3, "two": 4.2 },
-    "exp": "1.3 and 4.2"
-  },
-  {
-    "src": "{$one} et {$two}",
-    "locale": "fr",
-    "params": { "one": 1.3, "two": 4.2 },
-    "exp": "1,3 et 4,2"
-  },
-  { "src": ".local $foo = {bar} {{bar {$foo}}}", "exp": "bar bar" },
-  { "src": ".local $foo = {|bar|} {{bar {$foo}}}", "exp": "bar bar" },
-  {
-    "src": ".local $foo = {|bar|} {{bar {$foo}}}",
-    "params": { "foo": "foo" },
-    "exp": "bar bar"
-  },
-  {
-    "src": ".local $foo = {$bar} {{bar {$foo}}}",
-    "params": { "bar": "foo" },
-    "exp": "bar foo"
-  },
-  {
-    "src": ".local $foo = {$baz} .local $bar = {$foo} {{bar {$bar}}}",
-    "params": { "baz": "foo" },
-    "exp": "bar foo"
-  },
-  {
-    "src": ".input {$foo} {{bar {$foo}}}",
-    "params": { "foo": "foo" },
-    "exp": "bar foo"
-  },
-  {
-    "src": ".input {$foo} .local $bar = {$foo} {{bar {$bar}}}",
-    "params": { "foo": "foo" },
-    "exp": "bar foo"
-  },
-  {
-    "src": ".local $foo = {$baz} .local $bar = {$foo} {{bar {$bar}}}",
-    "params": { "baz": "foo" },
-    "exp": "bar foo"
-  },
-  { "src": ".local $x = {42} .local $y = {$x} {{{$x} {$y}}}", "exp": "42 42" },
-  {
-    "src": "{#tag}",
-    "exp": "",
-    "parts": [{ "type": "markup", "kind": "open", "name": "tag" }]
-  },
-  {
-    "src": "{#tag}content",
-    "exp": "content",
-    "parts": [
-      { "type": "markup", "kind": "open", "name": "tag" },
-      { "type": "literal", "value": "content" }
-    ]
-  },
-  {
-    "src": "{#ns:tag}content{/ns:tag}",
-    "exp": "content",
-    "parts": [
-      { "type": "markup", "kind": "open", "name": "ns:tag" },
-      { "type": "literal", "value": "content" },
-      { "type": "markup", "kind": "close", "name": "ns:tag" }
-    ]
-  },
-  {
-    "src": "{/tag}content",
-    "exp": "content",
-    "parts": [
-      { "type": "markup", "kind": "close", "name": "tag" },
-      { "type": "literal", "value": "content" }
-    ]
-  },
-  {
-    "src": "{#tag foo=bar}",
-    "exp": "",
-    "parts": [
-      {
-        "type": "markup",
-        "kind": "open",
-        "name": "tag",
-        "options": { "foo": "bar" }
-      }
-    ]
-  },
-  {
-    "src": "{#tag foo=bar/}",
-    "cleanSrc": "{#tag foo=bar /}",
-    "exp": "",
-    "parts": [
-      {
-        "type": "markup",
-        "kind": "standalone",
-        "name": "tag",
-        "options": { "foo": "bar" }
-      }
-    ]
-  },
-  {
-    "src": "{#tag a:foo=|foo| b:bar=$bar}",
-    "params": { "bar": "b a r" },
-    "exp": "",
-    "parts": [
-      {
-        "type": "markup",
-        "kind": "open",
-        "name": "tag",
-        "options": { "a:foo": "foo", "b:bar": "b a r" }
-      }
-    ]
-  },
-  {
-    "src": "{/tag foo=bar}",
-    "exp": "",
-    "parts": [
-      {
-        "type": "markup",
-        "kind": "close",
-        "name": "tag",
-        "options": { "foo": "bar" }
-      }
-    ]
-  },
-  {
-    "src": "{42 @foo @bar=13}",
-    "exp": "42",
-    "parts": [{ "type": "string", "value": "42" }]
-  },
-  {
-    "src": "{42 @foo=$bar}",
-    "exp": "42",
-    "parts": [{ "type": "string", "value": "42" }]
-  },
-  {
-    "src": "foo {+reserved}",
-    "exp": "foo {+}",
-    "parts": [
-      { "type": "literal", "value": "foo " },
-      { "type": "fallback", "source": "+" }
-    ],
-    "errorsJs": [{ "type": "unsupported-annotation" }]
-  },
-  {
-    "src": "foo {&private}",
-    "exp": "foo {&}",
-    "parts": [
-      { "type": "literal", "value": "foo " },
-      { "type": "fallback", "source": "&" }
-    ],
-    "errorsJs": [{ "type": "unsupported-annotation" }]
-  },
-  {
-    "src": "foo {?reserved @a @b=$c}",
-    "exp": "foo {?}",
-    "parts": [
-      { "type": "literal", "value": "foo " },
-      { "type": "fallback", "source": "?" }
-    ],
-    "errorsJs": [{ "type": "unsupported-annotation" }]
-  },
-  {
-    "src": ".foo {42} {{bar}}",
-    "exp": "bar",
-    "parts": [{ "type": "literal", "value": "bar" }],
-    "errorsJs": [{ "type": "unsupported-statement" }]
-  },
-  {
-    "src": ".foo{42}{{bar}}",
-    "cleanSrc": ".foo {42} {{bar}}",
-    "exp": "bar",
-    "parts": [{ "type": "literal", "value": "bar" }],
-    "errorsJs": [{ "type": "unsupported-statement" }]
-  },
-  {
-    "src": ".foo |}lit{| {42}{{bar}}",
-    "cleanSrc": ".foo |}lit{| {42} {{bar}}",
-    "exp": "bar",
-    "parts": [{ "type": "literal", "value": "bar" }],
-    "errorsJs": [{ "type": "unsupported-statement" }]
-  }
-]
diff --git a/icu4j/pom.xml b/icu4j/pom.xml
index a17662603fd0..eeee1cb8c3fc 100644
--- a/icu4j/pom.xml
+++ b/icu4j/pom.xml
@@ -279,6 +279,7 @@
                     <directory>${rootlocation}/..</directory>
                     <includes>
                       <include>LICENSE</include>
+                      <include>testdata/</include>
                     </includes>
                   </resource>
                 </resources>
diff --git a/testdata/message2/README.txt b/testdata/message2/README.txt
new file mode 100644
index 000000000000..17c7c656c320
--- /dev/null
+++ b/testdata/message2/README.txt
@@ -0,0 +1,88 @@
+© 2024 and later: Unicode, Inc. and others.
+License & terms of use: http://www.unicode.org/copyright.html
+
+The format of the JSON files in this directory follows the same format as `test-core.json`
+in the spec, described in:
+
+https://github.com/unicode-org/message-format-wg/blob/main/test/README.md
+
+The `parts` field is not used.
+
+# JSON extensions
+
+An additional `comment` field may be present, which is only for human readers.
+
+A "srcs" field, whose value is an array of strings, may be present instead
+of "src". The strings are concatenated to get the message.
+
+In the "params" field, a date parameter can be expressed as:
+{ "date": n }
+where n is a number representing a Unix timestamp.
+
+In the "params" field, a decimal string parameter can be expressed as:
+{ "decimal": s }
+where s is a string.
+
+Optional fields, "ignoreJava" and "ignoreCpp" can be used
+for tests currently expected to fail in the respective language.
+The field may have any value; if it's
+present, the test is ignored. (The value can be a comment explaining
+why it's expected to fail.)
+
+Tests in the `spec/` subdirectory are taken from https://github.com/unicode-org/message-format-wg/blob/main/test .
+If the contents change upstream, then the corresponding tests in CLDR
+need to be updated (also see https://unicode-org.atlassian.net/browse/ICU-22812 ).
+
+## ICU4J only
+
+The `cleanSrc` fields is used to represent normalized input (ICU4C has its
+own function for normalizing input).
+
+## ICU4C only
+
+Additional "char" and "line" fields may be present with integer values,
+used for tests expected to trigger a syntax error.
+If present, "char" reflects the expected character offset and "line"
+reflects the expected line number in the parse error.
+The files with "diagnostics" in the name have these fields filled in.
+
+# ICU4C vs. ICU4J tests
+
+The following tests are run in both ICU4C and ICU4J:
+
+* alias-selector-annotations.json
+* duplicate-declarations.json
+* icu-parser-tests.json
+  - Two tests removed while single-sourcing tests, because a `{{}}` message body
+  had to be added to get it to parse in ICU4C, and this broke the test in ICU4J.
+  These tests are in icu-parser-tests-old.json
+* icu-test-functions.json
+  - Some tests marked as ignored
+* icu-test-previous-release.json
+  - Some tests marked as ignored
+* icu-test-selectors.json
+* markup.json
+* matches-whitespace.json
+  - Some tests marked as ignored
+* more-data-model-errors.json
+* more-syntax-errors.json
+* reserved-syntax.json
+  - All tests marked as ignored in Java (resolution errors are suppressed)
+* resolution-errors.json
+  - All tests marked as ignored in Java (resolution errors are suppressed)
+* runtime-errors.json
+  - All tests marked as ignored in Java (message function errors are suppressed)
+* syntax-errors-diagnostics.json
+* tricky-declarations.json
+* valid-tests.json
+  - Some tests marked as ignored
+* spec/*
+  - Some tests in test-core.json and test-functions.json marked as ignored
+
+The following tests are only run in ICU4C, either because ICU4J doesn't check
+for invalid options, or because ICU4J doesn't report line/column numbers for
+parse errors:
+* invalid-number-literals-diagnostics.json
+* invalid-options.json
+* syntax-errors-diagnostics-multiline.json
+* syntax-errors-end-of-input.json
diff --git a/icu4c/source/test/testdata/message2/alias-selector-annotations.json b/testdata/message2/alias-selector-annotations.json
similarity index 100%
rename from icu4c/source/test/testdata/message2/alias-selector-annotations.json
rename to testdata/message2/alias-selector-annotations.json
diff --git a/icu4c/source/test/testdata/message2/duplicate-declarations.json b/testdata/message2/duplicate-declarations.json
similarity index 100%
rename from icu4c/source/test/testdata/message2/duplicate-declarations.json
rename to testdata/message2/duplicate-declarations.json
diff --git a/icu4c/source/test/testdata/message2/icu4j/icu-parser-tests.json b/testdata/message2/icu-parser-tests.json
similarity index 98%
rename from icu4c/source/test/testdata/message2/icu4j/icu-parser-tests.json
rename to testdata/message2/icu-parser-tests.json
index b9eb24d2330f..0cdfe0a467aa 100644
--- a/icu4c/source/test/testdata/message2/icu4j/icu-parser-tests.json
+++ b/testdata/message2/icu-parser-tests.json
@@ -55,7 +55,7 @@
     ".something |reserved=| {$foo :date} {{}}"
   ],
   "Multiple declarations in one message": [
-    ".input {$a :date} .local $b = {$a :date year=numeric month=long day=numeric} .local $c = {$b :date month=medium} .someting |reserved = \\| and more| {$x :date} {$y :date} {$z :number} {{}}",
-    ".input {$a :date} .local $exp = {$a :date style=full} {{Your card expires on {$exp}!}}"
+    ".input {$a :date} .local $exp = {$a :date style=full} {{Your card expires on {$exp}!}}",
+    ".input {$a :date} .local $b = {$a :date year=numeric month=long day=numeric} .local $c = {$b :date month=medium} .someting |reserved = \\| and more| {$x :date} {$y :date} {$z :number} {{}}"
   ]
 }
diff --git a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-test-functions.json b/testdata/message2/icu-test-functions.json
similarity index 66%
rename from icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-test-functions.json
rename to testdata/message2/icu-test-functions.json
index 9dc118bd07df..2dfd91cb8030 100644
--- a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-test-functions.json
+++ b/testdata/message2/icu-test-functions.json
@@ -2,71 +2,91 @@
   "Date and time formats": [
     {
       "src": "Expires on {$exp}",
-      "exp": "Expires on 8/3/24, 9:43 PM"
+      "exp": "Expires on 8/3/24, 9:43 PM",
+      "comment": "Modified from ICU4J copy to add params (likewise with the other date/time tests); 1722746637000 is 2024-08-03 21:43:57 PDT",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime}",
-      "exp": "Expires on 8/3/24, 9:43 PM"
+      "exp": "Expires on 8/3/24, 9:43 PM",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime icu:skeleton=yMMMMdjmsSSEE}",
-      "exp": "Expires on Sat, August 3, 2024 at 9:43:57.00 PM"
+      "exp": "Expires on Sat, August 3, 2024 at 9:43:57.00 PM",
+      "params": {"exp": { "date": 1722746637000 } },
+      "ignoreCpp": "ICU-22754 Skeleton option not implemented yet"
     },
     {
       "src": "Expires on {$exp :datetime dateStyle=full}",
-      "exp": "Expires on Saturday, August 3, 2024"
+      "exp": "Expires on Saturday, August 3, 2024",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime dateStyle=long}",
-      "exp": "Expires on August 3, 2024"
+      "exp": "Expires on August 3, 2024",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime dateStyle=medium}",
-      "exp": "Expires on Aug 3, 2024"
+      "exp": "Expires on Aug 3, 2024",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime timeStyle=long}",
-      "exp": "Expires on 9:43:57 PM PDT"
+      "exp": "Expires on 9:43:57 PM PDT",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime timeStyle=medium}",
-      "exp": "Expires on 9:43:57 PM"
+      "exp": "Expires on 9:43:57 PM",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime timeStyle=short}",
-      "exp": "Expires on 9:43 PM"
+      "exp": "Expires on 9:43 PM",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime dateStyle=full timeStyle=medium}",
-      "exp": "Expires on Saturday, August 3, 2024 at 9:43:57 PM"
+      "exp": "Expires on Saturday, August 3, 2024 at 9:43:57 PM",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime year=numeric month=long}",
-      "exp": "Expires on August 2024"
+      "exp": "Expires on August 2024",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "src": "Expires on {$exp :datetime year=numeric month=medium day=numeric weekday=long hour=numeric minute=numeric}",
-      "exp": "Expires on 3 Saturday 2024, 9:43 PM"
+      "exp": "Expires on 3 Saturday 2024, 9:43 PM",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "comment": "Make sure we ignore date / time fields if needed",
       "src": "Expires on {$exp :date year=numeric month=medium day=numeric weekday=long hour=numeric minute=numeric}",
-      "exp": "Expires on 3 Saturday 2024"
+      "exp": "Expires on 3 Saturday 2024",
+      "params": {"exp": { "date": 1722746637000 } },
+      "ignoreCpp": "ICU-22754 ICU4C doesn't accept field options for `:date` or `:time` -- see spec"
     },
     {
       "comment": "Make sure we ignore date / time fields if needed",
       "src": "Expires at {$exp :time year=numeric month=medium day=numeric weekday=long hour=numeric minute=numeric}",
-      "exp": "Expires at 9:43 PM"
+      "exp": "Expires at 9:43 PM",
+      "params": {"exp": { "date": 1722746637000 } },
+      "ignoreCpp": "ICU-22754 ICU4C doesn't accept field options for `:date` or `:time` -- see spec"
     },
     {
       "comment": "Make sure we ignore date / time fields if needed",
       "src": "Expires at {$exp :time style=long dateStyle=full timeStyle=medium}",
-      "exp": "Expires at 9:43:57 PM PDT"
+      "exp": "Expires at 9:43:57 PM PDT",
+      "params": {"exp": { "date": 1722746637000 } }
     },
     {
       "comment": "Make sure we ignore date / time fields if needed",
       "src": "Expires on {$exp :date style=long dateStyle=full timeStyle=medium}",
-      "exp": "Expires on August 3, 2024"
+      "exp": "Expires on August 3, 2024",
+      "params": {"exp": { "date": 1722746637000 } }
     }
   ],
   "Literals" : [
@@ -88,11 +108,13 @@
     },
     {
       "src": "Expires at {|2024-07-02T19:23:45Z| :datetime timeStyle=long}",
-      "exp": "Expires at 7:23:45 PM GMT"
+      "exp": "Expires at 7:23:45 PM GMT",
+      "ignoreCpp": "ICU-22754 Time zones not working yet (bug)"
     },
     {
       "src": "Expires at {|2024-07-02T19:23:45+03:30| :datetime timeStyle=full}",
-      "exp": "Expires at 7:23:45 PM GMT+03:30"
+      "exp": "Expires at 7:23:45 PM GMT+03:30",
+      "ignoreCpp": "ICU-22754 Time zones not working yet (bug)"
     }
   ],
   "Chaining" : [
@@ -105,7 +127,9 @@
         ".local $zooExp = {$exp :datetime dateStyle=short timeStyle=$tsOver}\n",
         "{{Hello John, you want '{$exp}', '{$longExp}', or '{$zooExp}' or even '{$exp :datetime dateStyle=full}'?}}"
       ],
-      "exp": "Hello John, you want '9:43 PM', 'August 3, 2024 at 9:43 PM', or '8/3/24, 9:43:57 PM Pacific Daylight Time' or even 'Saturday, August 3, 2024 at 9:43 PM'?"
+      "exp": "Hello John, you want '9:43 PM', 'August 3, 2024 at 9:43 PM', or '8/3/24, 9:43:57 PM Pacific Daylight Time' or even 'Saturday, August 3, 2024 at 9:43 PM'?",
+      "params": {"exp": { "date": 1722746637000 }, "user": "John", "tsOver" : "full" },
+      "ignoreCpp": "ICU-22754 ICU4C doesn't implement this kind of function composition yet. See https://github.com/unicode-org/message-format-wg/issues/515"
     },
     {
       "srcs": [
@@ -113,7 +137,8 @@
         ".local $longExp = {$exp :datetime month=long weekday=long}\n",
         "{{Expires on '{$exp}' ('{$longExp}').}}"
       ],
-      "exp": "Expires on '8/03/2024' ('Saturday, August 03, 2024')."
+      "exp": "Expires on '8/03/2024' ('Saturday, August 03, 2024').",
+      "params": {"exp": { "date": 1722746637000 } }
     }
   ],
   "Number formatter" : [
@@ -125,7 +150,8 @@
     {
       "src": "Format {123456789.9876} number",
       "locale": "en-IN",
-      "exp": "Format 12,34,56,789.9876 number"
+      "exp": "Format 12,34,56,789.9876 number",
+      "ignoreCpp": "ICU-22754 No default formatting for numbers, so it's formatted as a literal string. Is this in the spec?"
     },
     {
       "src": "Format {|3.1416|} number",
@@ -135,7 +161,8 @@
     {
       "src": "Format {|3.1416|} number",
       "locale": "ar-AR-u-nu-arab",
-      "exp": "Format ٣٫١٤١٦ number"
+      "exp": "Format ٣٫١٤١٦ number",
+      "ignoreCpp": "ICU-22754 No default formatting for numbers, so it's formatted as a literal string. Is this in the spec?"
     },
     {
       "src": "Format {3.1415926 :number}",
diff --git a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-test-previous-release.json b/testdata/message2/icu-test-previous-release.json
similarity index 70%
rename from icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-test-previous-release.json
rename to testdata/message2/icu-test-previous-release.json
index 6c235c2a3add..cc863411db95 100644
--- a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-test-previous-release.json
+++ b/testdata/message2/icu-test-previous-release.json
@@ -19,7 +19,8 @@
     {
       "src": "hello {$place}",
       "exp": "hello {$place}",
-      "errorsJs": [{ "type": "missing-var" }]
+      "errors": [{ "type": "unresolved-var" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": "{$one} and {$two}",
@@ -43,13 +44,15 @@
     },
     {
       "src": "hello {|foo| :number}",
-      "exp": "hello {|foo|}"
+      "exp": "hello {|foo|}",
+      "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": "hello {:number}",
-      "exp": "hello {|foo|}",
+      "exp": "hello {:number}",
       "comment": "This is different from JS, should be an error.",
-      "errors": [{ "type": "ICU4J: exception" }]
+      "errors": [{ "type": "bad-input" }]
     },
     {
       "src": "hello {|4.2| :number minimumFractionDigits=2}",
@@ -61,7 +64,7 @@
     },
     {
       "src": "hello {|4.2| :number minimumFractionDigits=$foo}",
-      "params": { "foo": 2f },
+      "params": { "foo": 2.0 },
       "exp": "hello 4.20"
     },
     {
@@ -97,16 +100,18 @@
       "exp": "bar 4.20"
     },
     {
-      "ignore": "Maybe. Because `minimumFractionDigits=foo`",
+      "ignoreJava": "Maybe. Because `minimumFractionDigits=foo`",
       "src": ".local $foo = {$bar :number minimumFractionDigits=foo} {{bar {$foo}}}",
       "params": { "bar": 4.2 },
       "exp": "bar 4.2",
-      "errors": [{ "type": "invalid-type" }]
+      "errors": [{ "type": "bad-option" }]
     },
     {
       "src": ".local $foo = {$bar :number} {{bar {$foo}}}",
       "params": { "bar": "foo" },
-      "exp": "bar {|foo|}"
+      "exp": "bar {$bar}",
+      "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": ".local $bar = {$baz} .local $foo = {$bar} {{bar {$foo}}}",
@@ -122,9 +127,8 @@
     {
       "src": ".match {$foo :number} 1 {{one}} * {{other}}",
       "params": { "foo": "1" },
-      "comment": "Should this be error? Plural on string?",
-      "expectedJs": "one",
-      "exp": "other"
+      "exp": "one",
+      "ignoreJava": "See ICU-22809"
     },
     {
       "src": ".match {$foo :string} 1 {{one}} * {{other}}",
@@ -143,7 +147,7 @@
       "exp": "one"
     },
     {
-      "ignore": "Can't pass null in a map",
+      "ignoreJava": "Can't pass null in a map",
       "src": ".match {$foo} 1 {{one}} * {{other}}",
       "params": { "foo": null },
       "exp": "other"
@@ -210,25 +214,11 @@
     },
     {
       "srcJs": ".local $bar = {$none} .match {$foo} one {{one}} * {{{$bar}}}",
-      "src": ".local $bar = {$none :number} .match {$foo} one {{one}} * {{{$bar}}}",
+      "src": ".local $bar = {$none :number} .match {$foo :string} one {{one}} * {{{$bar}}}",
       "params": { "foo": 2 },
-      "exp": "{$bar}",
-      "errors": [{ "type": "missing-var" }]
-    },
-    {
-      "src": ".local bar = {|foo|} {{$bar}}",
-      "exp": "{$bar}",
-      "errors": [{ "type": "missing-char" }, { "type": "missing-var" }]
-    },
-    {
-      "src": ".local $bar {|foo|} {{$bar}}",
-      "exp": "foo",
-      "errors": [{ "type": "missing-char" }]
-    },
-    {
-      "src": ".local $bar = |foo| {{$bar}}",
-      "exp": "{$bar}",
-      "errors": [{ "type": "missing-char" }, { "type": "junk-element" }]
+      "exp": "{$none}",
+      "errors": [{ "type": "unresolved-var" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": "{{#tag}}",
@@ -281,25 +271,11 @@
       "exp": "no braces 2",
       "errorsJs": [{ "type":  "parse-error" }, { "type": "junk-element" }]
     },
-    {
-      "src": "{missing end brace",
-      "exp": "missing end brace",
-      "errors": [{ "type": "missing-char" }]
-    },
-    {
-      "src": "{missing end {$brace",
-      "exp": "missing end {$brace}",
-      "errors": [{ "type": "missing-char" }, { "type": "missing-char" }, { "type": "missing-var" }]
-    },
-    {
-      "src": "{{extra}} content",
-      "exp": "extra",
-      "errors": [{ "type": "extra-content" }]
-    },
     {
       "src": "empty { }",
       "exp": "empty ",
-      "errors": [{ "type": "parse-error" }, { "type": "junk-element" }]
+      "errors": [{ "type": "parse-error" }, { "type": "junk-element" }],
+      "ignoreCpp": "Fallback is unclear. See https://github.com/unicode-org/message-format-wg/issues/703"
     },
     {
       "src": "bad {:}",
@@ -311,80 +287,27 @@
       "exp": "bad placeholder",
       "errorsJs": [{ "type": "parse-error" }, { "type": "extra-content" }, { "type": "junk-element" }]
     },
-    {
-      "src": "no-equal {|42| :number minimumFractionDigits 2}",
-      "exp": "no-equal 42.00",
-      "errors": [{ "type": "missing-char" }]
-    },
-    {
-      "src": "bad {:placeholder option=}",
-      "exp": "bad {:placeholder}",
-      "errors": [{ "type": "empty-token" }, { "type": "missing-func" }]
-    },
-    {
-      "src": "bad {:placeholder option value}",
-      "exp": "bad {:placeholder}",
-      "errors": [{ "type": "missing-char" }, { "type": "missing-func" }]
-    },
-    {
-      "src": "{bad {:placeholder option}}",
-      "exp": "bad {:placeholder}",
-      "errors": [{ "type": "missing-char" }, { "type": "empty-token" }, { "type": "missing-func" }]
-    },
     {
       "src": "{bad {$placeholder option}}",
       "exp": "bad {$placeholder}",
-      "errors": [{ "type": "extra-content" }, { "type": "extra-content" }, { "type": "missing-var" }]
-    },
-    {
-      "src": "{no {$placeholder end}",
-      "exp": "no {$placeholder}",
-      "errors": [{ "type": "extra-content" }, { "type": "missing-var" }]
-    },
-    {
-      "src": ".match {} * {{foo}}",
-      "exp": "foo",
-      "errors": [{ "type": "parse-error" }, { "type": "bad-selector" }, { "type": "junk-element" }]
-    },
-    {
-      "src": ".match {#foo} * {{foo}}",
-      "exp": "foo",
-      "errors": [{ "type": "bad-selector" }]
+      "errors": [{ "type": "extra-content" }, { "type": "extra-content" }, { "type": "missing-var" }],
+      "ignoreCpp": "Fallback is unclear. See https://github.com/unicode-org/message-format-wg/issues/703"
     },
     {
-      "src": ".match {|foo|} *{{foo}}",
-      "exp": "foo",
-      "errors": [{ "type": "missing-char" }]
-    },
-    {
-      "src": ".match * {{foo}}",
-      "exp": "foo",
-      "errors": [{ "type": "empty-token" }]
-    },
-    {
-      "src": ".match {|x|} * foo",
-      "exp": "",
-      "errors": [{ "type": "key-mismatch" }, { "type": "missing-char" }]
-    },
-    {
-      "src": ".match {|x|} * {{foo}} extra",
-      "exp": "foo",
-      "errors": [{ "type": "extra-content" }]
-    },
-    {
-      "src": ".match |x| * {{foo}}",
-      "exp": "",
-      "errors": [{ "type": "empty-token" }, { "type": "extra-content" }]
+      "src": ".match {|foo| :string} *{{foo}}",
+      "exp": "foo"
     },
     {
-      "src": ".match {$foo} * * {{foo}}",
+      "src": ".match {$foo :string} * * {{foo}}",
       "exp": "foo",
-      "errors": [{ "type": "key-mismatch" }, { "type": "missing-var" }]
+      "errors": [{ "type": "key-mismatch" }, { "type": "missing-var" }],
+      "ignoreCpp": "Fallback is unclear. See https://github.com/unicode-org/message-format-wg/issues/735"
     },
     {
-      "src": ".match {$foo} {$bar} * {{foo}}",
+      "src": ".match {$foo :string} {$bar :string} * {{foo}}",
       "exp": "foo",
-      "errors": [{ "type": "key-mismatch" }, { "type": "missing-var" }, { "type": "missing-var" }]
+      "errors": [{ "type": "key-mismatch" }, { "type": "missing-var" }, { "type": "missing-var" }],
+      "ignoreCpp": "Fallback is unclear. See https://github.com/unicode-org/message-format-wg/issues/735"
     }
 ]
 
diff --git a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-test-selectors.json b/testdata/message2/icu-test-selectors.json
similarity index 98%
rename from icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-test-selectors.json
rename to testdata/message2/icu-test-selectors.json
index 1f237254f198..efe67e7c4c9e 100644
--- a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/icu-test-selectors.json
+++ b/testdata/message2/icu-test-selectors.json
@@ -48,7 +48,7 @@
         "1   {{You got the gold medal}}\n",
         "2   {{You got the silver medal}}\n",
         "3   {{You got the bronze medal}}\n",
-        "few {{You finished in the {$place}rd place}}\n"
+        "few {{You finished in the {$place}rd place}}"
       ]
     },
     "variations" : [
@@ -70,7 +70,7 @@
         "  *   *   {{You found {$fileCount} files in {$folderCount} folders}}\n",
         "  one one {{You found {$fileCount} file in {$folderCount} folder}}\n",
         "  one *   {{You found {$fileCount} file in {$folderCount} folders}}\n",
-        "  *   one {{You found {$fileCount} files in {$folderCount} folder}}\n"
+        "  *   one {{You found {$fileCount} files in {$folderCount} folder}}"
       ]
     },
     "variations" : [
diff --git a/icu4c/source/test/testdata/message2/invalid-number-literals-diagnostics.json b/testdata/message2/invalid-number-literals-diagnostics.json
similarity index 100%
rename from icu4c/source/test/testdata/message2/invalid-number-literals-diagnostics.json
rename to testdata/message2/invalid-number-literals-diagnostics.json
diff --git a/testdata/message2/invalid-options.json b/testdata/message2/invalid-options.json
new file mode 100644
index 000000000000..8b4740ee9bcb
--- /dev/null
+++ b/testdata/message2/invalid-options.json
@@ -0,0 +1,34 @@
+[
+    { "src": ".local $foo = {1 :number minimumIntegerDigits=-1} {{bar {$foo}}}",
+      "errors": [{"type": "bad-option"}],
+      "ignoreJava": "ICU4J doesn't validate options"
+    },
+    { "src": ".local $foo = {1 :number minimumIntegerDigits=foo} {{bar {$foo}}}",
+      "errors": [{"type": "bad-option"}],
+      "ignoreJava": "ICU4J doesn't validate options"
+    },
+    { "src": ".local $foo = {1 :number minimumFractionDigits=foo} {{bar {$foo}}}",
+      "errors": [{"type": "bad-option"}],
+      "ignoreJava": "ICU4J doesn't validate options"
+    },
+    { "src": ".local $foo = {1 :number maximumFractionDigits=foo} {{bar {$foo}}}",
+      "errors": [{"type": "bad-option"}],
+      "ignoreJava": "ICU4J doesn't validate options"
+    },
+    { "src": ".local $foo = {1 :number minimumSignificantDigits=foo} {{bar {$foo}}}",
+      "errors": [{"type": "bad-option"}],
+      "ignoreJava": "ICU4J doesn't validate options"
+    },
+    { "src": ".local $foo = {1 :number maximumSignificantDigits=foo} {{bar {$foo}}}",
+      "errors": [{"type": "bad-option"}],
+      "ignoreJava": "ICU4J doesn't validate options"
+    },
+    { "src": ".local $foo = {1 :integer minimumIntegerDigits=foo} {{bar {$foo}}}",
+      "errors": [{"type": "bad-option"}],
+      "ignoreJava": "ICU4J doesn't validate options"
+    },
+    { "src": ".local $foo = {1 :integer maximumSignificantDigits=foo} {{bar {$foo}}}",
+      "errors": [{"type": "bad-option"}],
+      "ignoreJava": "ICU4J doesn't validate options"
+    }
+]
diff --git a/icu4c/source/test/testdata/message2/markup.json b/testdata/message2/markup.json
similarity index 100%
rename from icu4c/source/test/testdata/message2/markup.json
rename to testdata/message2/markup.json
diff --git a/icu4c/source/test/testdata/message2/matches-whitespace.json b/testdata/message2/matches-whitespace.json
similarity index 82%
rename from icu4c/source/test/testdata/message2/matches-whitespace.json
rename to testdata/message2/matches-whitespace.json
index c70bef04b650..d0b2c4ecdfe7 100644
--- a/icu4c/source/test/testdata/message2/matches-whitespace.json
+++ b/testdata/message2/matches-whitespace.json
@@ -2,19 +2,25 @@
     { "src": ".match {one :string} {bar :string} one * {{one}} * * {{other}}",
       "exp": "one" },
     { "src": ".match {foo :string} {bar :string}one * {{one}} * * {{other}}",
-      "exp": "other" },
+      "exp": "other"
+    },
     { "src": ".match {foo :string}{bar :string} one * {{one}} * * {{other}}",
-      "exp": "other" },
+      "exp": "other"
+    },
     { "src": ".match {one :string}{bar :string}one * {{one}} * * {{other}}",
-      "exp": "one" },
+      "exp": "one"
+    },
     { "src": ".match{foo :string} {bar :string} one * {{one}} * * {{other}}",
-      "exp": "other" },
+      "exp": "other"
+    },
     { "src": ".match {foo :string} {bar :string} one * {{one}}* * {{other}}",
       "exp": "other" },
     { "src": ".match {foo :string} {bar :string}one * {{one}}* * {{other}}",
-      "exp": "other" },
+      "exp": "other"
+    },
     { "src": ".match {foo :string} {bar :string} one *{{one}} * * {{foo}}",
-      "exp": "foo" },
+      "exp": "foo"
+    },
     { "src": ".match {foo :string} {bar :string} one *  {{one}} * * {{foo}}",
       "exp": "foo" }
 ]
diff --git a/icu4c/source/test/testdata/message2/more-data-model-errors.json b/testdata/message2/more-data-model-errors.json
similarity index 94%
rename from icu4c/source/test/testdata/message2/more-data-model-errors.json
rename to testdata/message2/more-data-model-errors.json
index ee26a1ffdf2f..0c86c58a5a12 100644
--- a/icu4c/source/test/testdata/message2/more-data-model-errors.json
+++ b/testdata/message2/more-data-model-errors.json
@@ -30,9 +30,5 @@
         "{:foo a=1 a=1}",
         "{:foo a=1 a=2}",
         "{|x| :foo a=1 a=2}"
-    ],
-    "Unsupported Statement": [
-        ".matc {-1} {{hello}}",
-        ".m {-1} {{hello}}"
     ]
 }
diff --git a/icu4c/source/test/testdata/message2/more-functions.json b/testdata/message2/more-functions.json
similarity index 87%
rename from icu4c/source/test/testdata/message2/more-functions.json
rename to testdata/message2/more-functions.json
index 161379a63c55..99640a4daed3 100644
--- a/icu4c/source/test/testdata/message2/more-functions.json
+++ b/testdata/message2/more-functions.json
@@ -67,7 +67,8 @@
       "locale": "en-IN",
       "params": {"val": 1234567890.97531},
       "errors": [{"type": "bad-input"}],
-      "comment": "Should fail because number literals are not treated as localized numbers"
+      "comment": "Should fail because number literals are not treated as localized numbers",
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": "From literal: {|123456789.531| :number}!",
@@ -99,7 +100,16 @@
       "exp": "Default int64: 1.234.567.890.123.456.800!",
       "locale": "ro",
       "params": {"val": 1234567890123456789},
-      "comment": "Rounded due to JSON not supporting full 64-bit ints"
+      "comment": "Rounded due to JSON not supporting full 64-bit ints",
+      "ignoreJava": "See https://unicode-org.atlassian.net/browse/ICU-22754?focusedCommentId=175932"
+    },
+    {
+      "src": "Default int64: {$val}!",
+      "exp": "Default int64: 1.234.567.890.123.456.770!",
+      "locale": "ro",
+      "params": {"val": 1234567890123456789},
+      "comment": "Rounded due to JSON not supporting full 64-bit ints",
+      "ignoreCpp": "See https://unicode-org.atlassian.net/browse/ICU-22754?focusedCommentId=175932"
     },
     {
       "src": "Default number: {$val}!",
diff --git a/testdata/message2/more-syntax-errors.json b/testdata/message2/more-syntax-errors.json
new file mode 100644
index 000000000000..48f145b452c0
--- /dev/null
+++ b/testdata/message2/more-syntax-errors.json
@@ -0,0 +1,5 @@
+[
+  ".input {$x :number}",
+  ".local $foo = {|1|}",
+  ".unsupported |statement| {$x :number}"
+]
diff --git a/testdata/message2/reserved-syntax.json b/testdata/message2/reserved-syntax.json
new file mode 100644
index 000000000000..9efd6fe6b9c9
--- /dev/null
+++ b/testdata/message2/reserved-syntax.json
@@ -0,0 +1,40 @@
+[
+    { "src": "hello {|4.2| %number}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations" },
+    { "src": "hello {|4.2| %n|um|ber}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "{+42}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| &num|be|r}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| ^num|be|r}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| +num|be|r}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| ?num|be||r|s}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|foo| !number}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|foo| *number}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {?number}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "{<tag}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": ".local $bar = {$none ~plural} .match {foo :string}  * {{{$bar}}}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| %num\\\\ber}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| %num\\{be\\|r}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| %num\\\\\\}ber}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| !}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| %}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| *}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| ^abc|123||5|\\\\}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| ^ abc|123||5|\\\\}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| ^ abc|123||5|\\\\ \\|def |3.14||2|}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| ? }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| %xyzz }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| >xyzz   }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {$foo ~xyzz }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {$x   <xyzz   }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "{>xyzz }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "{  !xyzz   }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "{~xyzz }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "{ <xyzz   }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| !xy z z }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| *num \\\\ b er}", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2| %num \\\\ b |3.14| r    }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {|4.2|    +num xx \\\\ b |3.14| r  }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {$foo    +num x \\\\ abcde |3.14| r  }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {$foo    >num x \\\\ abcde |aaa||3.14||42| r  }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  },
+    { "src": "hello {$foo    >num x \\\\ abcde |aaa||3.14| |42| r  }", "errors": [{ "type": "unsupported-annotation" }], "ignoreJava": "ICU4J doesn't error out on reserved annotations"  }
+]
+
diff --git a/testdata/message2/resolution-errors.json b/testdata/message2/resolution-errors.json
new file mode 100644
index 000000000000..077e8b0e9c3b
--- /dev/null
+++ b/testdata/message2/resolution-errors.json
@@ -0,0 +1,14 @@
+[
+    { "src": "{$oops}", "exp": "{$oops}", "errors": [{ "type": "unresolved-var" }], "ignoreJava": "ICU4J doesn't signal unresolved variable errors?"},
+    { "src": ".input {$x :number} {{{$x}}}", "exp": "{$x}", "errors": [{ "type": "unresolved-var" }], "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"},
+    { "src": ".local $foo = {$bar} .match {$foo :number}  one {{one}}  * {{other}}", "exp": "other", "errors": [{ "type": "unresolved-var" }], "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"},
+    { "src": ".local $bar = {$none :number} .match {$foo :string}  one {{one}}  * {{{$bar}}}", "exp": "{$none}", "errors": [{ "type": "unresolved-var" }], "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"},
+    { "src": "The value is {horse :func}.", "exp": "The value is {|horse|}.", "errors": [{ "type": "missing-func" }], "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"},
+    { "src": ".matc {-1} {{hello}}", 
+      "errors": [{ "type": "unsupported-statement" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
+    },
+    { "src": ".m {-1} {{hello}}", 
+      "errors": [{ "type": "unsupported-statement" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" }
+]
diff --git a/icu4c/source/test/testdata/message2/runtime-errors.json b/testdata/message2/runtime-errors.json
similarity index 61%
rename from icu4c/source/test/testdata/message2/runtime-errors.json
rename to testdata/message2/runtime-errors.json
index 5b93f54d7566..9c7bfbc58443 100644
--- a/icu4c/source/test/testdata/message2/runtime-errors.json
+++ b/testdata/message2/runtime-errors.json
@@ -2,21 +2,25 @@
     {
         "src": ".match {|horse| :date}\n   1 {{The value is one.}}\n  * {{Formatter used as selector.}}",
         "exp": "Formatter used as selector.",
-        "errors": [{"type": "selector-error"}]
+        "errors": [{"type": "selector-error"}],
+        "ignoreJava": "ICU4J doesn't signal runtime errors?"
     },
     {
         "src": ".match {|horse| :string}\n  1 {{The value is one.}}\n  * {{Selector used as formatter: {|horse| :string}}}",
         "exp": "Selector used as formatter: {|horse|}",
-        "errors": [{"type": "formatting-error"}]
+        "errors": [{"type": "formatting-error"}],
+        "ignoreJava": "ICU4J doesn't signal runtime errors?"
     },
     {
         "src": ".match {|horse| :number}\n  1 {{The value is one.}}\n  * {{horse is not a number.}}",
         "exp": "horse is not a number.",
-        "errors": [{"type": "selector-error"}]
+        "errors": [{"type": "selector-error"}],
+        "ignoreJava": "ICU4J doesn't signal runtime errors?"
     },
     {
         "src": ".local $sel = {|horse| :number}\n  .match {$sel}\n    1 {{The value is one.}}\n   * {{horse is not a number.}}",
         "exp": "horse is not a number.",
-        "errors": [{"type": "selector-error"}]
+        "errors": [{"type": "selector-error"}],
+        "ignoreJava": "ICU4J doesn't signal runtime errors?"
     }
 ]
diff --git a/icu4c/source/test/testdata/message2/spec/data-model-errors.json b/testdata/message2/spec/data-model-errors.json
similarity index 100%
rename from icu4c/source/test/testdata/message2/spec/data-model-errors.json
rename to testdata/message2/spec/data-model-errors.json
diff --git a/icu4c/source/test/testdata/message2/spec/syntax-errors.json b/testdata/message2/spec/syntax-errors.json
similarity index 100%
rename from icu4c/source/test/testdata/message2/spec/syntax-errors.json
rename to testdata/message2/spec/syntax-errors.json
diff --git a/icu4c/source/test/testdata/message2/spec/test-core.json b/testdata/message2/spec/test-core.json
similarity index 83%
rename from icu4c/source/test/testdata/message2/spec/test-core.json
rename to testdata/message2/spec/test-core.json
index 0e7049fdb1a3..2ee0786e62de 100644
--- a/icu4c/source/test/testdata/message2/spec/test-core.json
+++ b/testdata/message2/spec/test-core.json
@@ -26,7 +26,8 @@
   {
     "src": "hello {$place}",
     "errors": [{ "type": "unresolved-var" }],
-    "exp": "hello {$place}"
+    "exp": "hello {$place}",
+    "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
   },
   {
     "src": "{$one} and {$two}",
@@ -169,7 +170,8 @@
       { "type": "literal", "value": "foo " },
       { "type": "fallback", "source": "+" }
     ],
-    "errors": [{ "type": "unsupported-annotation" }]
+    "errors": [{ "type": "unsupported-annotation" }],
+    "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
   },
   {
     "src": "foo {&private}",
@@ -178,7 +180,8 @@
       { "type": "literal", "value": "foo " },
       { "type": "fallback", "source": "&" }
     ],
-    "errors": [{ "type": "unsupported-annotation" }]
+    "errors": [{ "type": "unsupported-annotation" }],
+    "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
   },
   {
     "src": "foo {?reserved @a @b=$c}",
@@ -187,26 +190,30 @@
       { "type": "literal", "value": "foo " },
       { "type": "fallback", "source": "?" }
     ],
-    "errors": [{ "type": "unsupported-annotation" }]
+    "errors": [{ "type": "unsupported-annotation" }],
+    "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
   },
   {
     "src": ".foo {42} {{bar}}",
     "exp": "bar",
     "parts": [{ "type": "literal", "value": "bar" }],
-    "errors": [{ "type": "unsupported-statement" }]
+    "errors": [{ "type": "unsupported-statement" }],
+    "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
   },
   {
     "src": ".foo{42}{{bar}}",
     "cleanSrc": ".foo {42} {{bar}}",
     "exp": "bar",
     "parts": [{ "type": "literal", "value": "bar" }],
-    "errors": [{ "type": "unsupported-statement" }]
+    "errors": [{ "type": "unsupported-statement" }],
+    "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
   },
   {
     "src": ".foo |}lit{| {42}{{bar}}",
     "cleanSrc": ".foo |}lit{| {42} {{bar}}",
     "exp": "bar",
     "parts": [{ "type": "literal", "value": "bar" }],
-    "errors": [{ "type": "unsupported-statement" }]
+    "errors": [{ "type": "unsupported-statement" }],
+    "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
   }
 ]
diff --git a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/test-functions.json b/testdata/message2/spec/test-functions.json
similarity index 77%
rename from icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/test-functions.json
rename to testdata/message2/spec/test-functions.json
index 8cf1b7a25325..a8979106947e 100644
--- a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/test/message2/test-functions.json
+++ b/testdata/message2/spec/test-functions.json
@@ -1,10 +1,16 @@
 {
   "date": [
-    { "src": "{:date}", "exp": "{:date}", "errorsJs": [{ "type": "bad-input" }] },
+    { "src": "{:date}",
+      "exp": "{:date}",
+      "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
+    },
     {
       "src": "{horse :date}",
       "exp": "{|horse|}",
-      "errorsJs": [{ "name": "RangeError" }]
+      "errors": [{ "type": "bad-input" }],
+      "errorsJs": [{ "name": "RangeError" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     { "src": "{|2006-01-02| :date}", "exp": "1/2/06" },
     { "src": "{|2006-01-02T15:04:06| :date}", "exp": "1/2/06" },
@@ -14,17 +20,20 @@
       "exp": "January 2, 2006"
     },
     {
-      "ignore": "Can't chain :time and :date, they are different types",
+      "ignoreJava": "Can't chain :time and :date, they are different types",
       "src": ".local $t = {|2006-01-02T15:04:06| :time} {{{$t :date}}}",
       "exp": "1/2/06"
     }
   ],
   "time": [
-    { "src": "{:time}", "exp": "{:time}", "errorsJs": [{ "type": "bad-input" }] },
+    { "src": "{:time}", "exp": "{:time}", "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
+    },
     {
       "src": "{horse :time}",
       "exp": "{|horse|}",
-      "errorsJs": [{ "name": "RangeError" }]
+      "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     { "src": "{|2006-01-02T15:04:06| :time}", "expJs": "3:04 PM", "exp": "3:04 PM" },
     {
@@ -38,16 +47,17 @@
       "exp": "3:04:06 PM"
     },
     {
-      "ignore": "Can't chain :time and :date, they are different types",
+      "ignoreJava": "Can't chain :time and :date, they are different types",
       "src": ".local $d = {|2006-01-02T15:04:06| :date} {{{$d :time}}}",
-      "exp": "3:04 PM"
+      "exp": "3:04 PM"
     }
   ],
   "datetime": [
     {
       "src": "{:datetime}",
       "exp": "{:datetime}",
-      "errorsJs": [{ "type": "bad-input" }]
+      "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": "{$x :datetime}",
@@ -58,7 +68,8 @@
     {
       "src": "{horse :datetime}",
       "exp": "{|horse|}",
-      "errorsJs": [{ "name": "RangeError" }]
+      "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     { "src": "{|2006-01-02T15:04:06| :datetime}", "expJs": "1/2/06, 3:04 PM", "exp": "1/2/06, 3:04 PM" },
     {
@@ -81,10 +92,11 @@
       "exp": "1/2/06, 3:04 PM"
     },
     {
-      "ignore": "Can't chain :time and :date, they are different types",
+      "ignoreJava": "Can't chain :time and :date, they are different types",
+      "ignoreCpp": "Same reason as Java",
       "src": ".input {$dt :time style=medium} {{{$dt :datetime dateStyle=long}}}",
       "params": { "dt": "2006-01-02T15:04:06" },
-      "exp": "January 2, 2006 at 3:04:06 PM"
+      "exp": "January 2, 2006 at 3:04:06 PM"
     }
   ],
   "integer": [
@@ -104,7 +116,8 @@
     {
       "src": "hello {foo :number}",
       "exp": "hello {|foo|}",
-      "errorsJs": [{ "type": "bad-input" }]
+      "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": "hello {:number}",
@@ -150,9 +163,9 @@
     {
       "src": ".local $foo = {$bar :number} {{bar {$foo}}}",
       "params": { "bar": "foo" },
-      "expJs": "bar {$bar}",
-      "exp": "bar {|foo|}",
-      "errorsJs": [{ "type": "bad-input" }]
+      "exp": "bar {$bar}",
+      "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": ".input {$foo :number} {{bar {$foo}}}",
@@ -175,9 +188,9 @@
     {
       "src": ".input {$foo :number} {{bar {$foo}}}",
       "params": { "foo": "foo" },
-      "expJs": "bar {$foo}",
-      "exp": "bar {|foo|}",
-      "errorsJs": [{ "type": "bad-input" }]
+      "exp": "bar {$foo}",
+      "errors": [{ "type": "bad-input" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": ".match {$foo :number} one {{one}} * {{other}}",
@@ -278,12 +291,14 @@
       "src": ".match {$foo :ordinal} one {{st}} two {{nd}} few {{rd}} * {{th}}",
       "params": { "foo": 1 },
       "exp": "th",
-      "errors": [{ "type": "missing-func" }, { "type": "not-selectable" }]
+      "errors": [{ "type": "missing-func" }, { "type": "not-selectable" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": "hello {42 :ordinal}",
       "exp": "hello {|42|}",
-      "errorsJs": [{ "type": "missing-func" }]
+      "errors": [{ "type": "missing-func" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     }
   ],
   "plural": [
@@ -291,12 +306,14 @@
       "src": ".match {$foo :plural} one {{one}} * {{other}}",
       "params": { "foo": 1 },
       "exp": "other",
-      "errors": [{ "type": "missing-func" }, { "type": "not-selectable" }]
+      "errors": [{ "type": "missing-func" }, { "type": "not-selectable" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     },
     {
       "src": "hello {42 :plural}",
       "exp": "hello {|42|}",
-      "errorsJs": [{ "type": "missing-func" }]
+      "errors": [{ "type": "missing-func" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     }
   ],
   "string": [
@@ -308,8 +325,8 @@
     {
       "src": ".match {$foo :string} 1 {{one}} * {{other}}",
       "params": { "foo": 1 },
-      "expJs": "one",
-      "exp": "other"
+      "exp": "one",
+      "ignoreJava": "See https://unicode-org.atlassian.net/browse/ICU-22754?focusedCommentId=175933"
     },
     {
       "src": ".match {$foo :string} 1 {{one}} * {{other}}",
@@ -319,7 +336,8 @@
     {
       "src": ".match {$foo :string} 1 {{one}} * {{other}}",
       "exp": "other",
-      "errorsJs": [{ "type": "unresolved-var" }]
+      "errors": [{ "type": "unresolved-var" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782"
     }
   ]
 }
diff --git a/icu4c/source/test/testdata/message2/syntax-errors-diagnostics-multiline.json b/testdata/message2/syntax-errors-diagnostics-multiline.json
similarity index 100%
rename from icu4c/source/test/testdata/message2/syntax-errors-diagnostics-multiline.json
rename to testdata/message2/syntax-errors-diagnostics-multiline.json
diff --git a/testdata/message2/syntax-errors-diagnostics.json b/testdata/message2/syntax-errors-diagnostics.json
new file mode 100644
index 000000000000..2b0188f6b557
--- /dev/null
+++ b/testdata/message2/syntax-errors-diagnostics.json
@@ -0,0 +1,402 @@
+[
+    { "src": "}{|xyz|", "char": 0, "errors": [{"type": "parse-error"}] },
+    { "src": "}", "char": 0, "errors": [{"type": "parse-error"}] },
+    {
+        "src": "{{{%\\y{}}",
+        "char": 5,
+        "comment": "Backslash followed by non-backslash followed by a '{' -- this should be an error immediately after the first backslash",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{%abc|\\z}}",
+        "char": 7,
+        "comment": "Reserved chars followed by a '|' that doesn't begin a valid literal -- this should be an error at the first invalid char in the literal",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{%\\y{p}}",
+        "char": 3,
+        "comment": "Same pattern, but with a valid reserved-char following the erroneous reserved-escape -- the offset should be the same as with the previous one",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{{{%ab|\\z|cd}}",
+        "char": 8,
+        "comment": "Erroneous literal inside a reserved string -- the error should be at the first erroneous literal char",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "hello {|4.2| %num\\ber}}",
+        "char": 18,
+        "comment": "Single backslash not allowed",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "hello {|4.2| %num{be\\|r}}",
+        "char": 17,
+        "comment": "Unescaped '{' not allowed",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "hello {|4.2| %num}be\\|r}}",
+        "char": 21,
+        "comment": "Unescaped '}' -- will be interpreted as the end of the reserved string, and the error will be reported at the index of '|', which is when the parser determines that the escaped '|' isn't a valid text-escape",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "hello {|4.2| %num\\{be|r}}",
+        "char": 25,
+        "comment": "Unescaped '|' -- will be interpreted as the beginning of a literal. Error at end of input",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match{|y|}|y|{{{|||}}}",
+        "char": 19,
+        "comment": "No spaces are required here. The error should be in the pattern, not before",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {|y|}|foo|bar {{{a}}}",
+        "char": 17,
+        "comment": "Missing spaces between keys",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {|y|} |quux| |foo|bar {{{a}}}",
+        "char": 25,
+        "comment": "Missing spaces between keys",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {|y|}  |quux| |foo||bar| {{{a}}}",
+        "char": 26,
+        "comment": "Missing spaces between keys",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {|y|}  |\\q| * %{! {z}",
+        "char": 16,
+        "comment": "Error parsing the first key -- the error should be there, not in the also-erroneous third key",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src":  ".match {|y|}  * %{! {z} |\\q|",
+        "char": 16,
+        "comment": "Error parsing the second key -- the error should be there, not in the also-erroneous third key",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {|y|}  * |\\q| {\\z}",
+        "char": 18,
+        "comment": "Error parsing the last key -- the error should be there, not in the erroneous pattern",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {|y|} {\\|} {@}  * * * {{a}}",
+        "char": 14,
+        "comment": "Non-expression as scrutinee in pattern -- error should be at the first non-expression, not the later non-expression",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {|y|}  $foo * {{a}} when * :bar {{b}}",
+        "char": 14,
+        "comment": "Non-key in variant -- error should be there, not in the next erroneous variant",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{{ foo {|bar|} \\q baz  ",
+        "char": 16,
+        "comment": "Error should be within the first erroneous `text` or expression",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{{{:    }}}",
+        "char": 4,
+        "comment": "':' has to be followed by a function name -- the error should be at the first whitespace character",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".local $x = }|foo|}",
+        "char": 12,
+        "comment": "Expression not starting with a '{'",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".local $x = {|foo|} .l $y = {|bar|} .local $z {|quux|}",
+        "char": 22,
+        "comment": "Error should be at the first declaration not starting with a `.local`",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".local $bar {|foo|} {{$bar}}",
+        "char": 12,
+        "comment": "Missing '=' in `.local` declaration",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".local bar = {|foo|} {{$bar}}",
+        "char": 7,
+        "comment": "LHS of declaration doesn't start with a '$'",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".local $bar = |foo| {{$bar}}",
+        "char": 14,
+        "comment": "`.local` RHS isn't an expression",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{{extra}}content",
+        "char": 9,
+        "comment": "Trailing characters that are not whitespace",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {|x|}  * {{foo}}extra",
+        "char": 28,
+        "comment": "Trailing characters that are not whitespace",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {$foo :string} {$bar :string}  one * {{one}}  * * {{other}}   ",
+        "char": 66,
+        "comment": "Trailing whitespace at end of message should not be accepted either",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{{hi}} ",
+        "char": 6,
+        "comment": "Trailing whitespace at end of message should not be accepted either",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "empty { }",
+        "char": 8,
+        "comment": "Empty expression",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {}  * {{foo}}",
+        "char": 8,
+        "comment": "Empty expression",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "bad {:}",
+        "char": 6,
+        "comment": "':' not preceding a function name",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{{no-equal {|42| :number m }}}",
+        "char":  27,
+        "comment": "Missing '=' after option name",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{{no-equal {|42| :number minimumFractionDigits 2}}}",
+        "char":  47,
+        "comment": "Missing '=' after option name",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "bad {:placeholder option value}",
+        "char":  25,
+        "comment": "Missing '=' after option name",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "hello {|4.2| :number min=2=3}",
+        "char":  26,
+        "comment": "Extra '=' after option name",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "hello {|4.2| :number min=2max=3}",
+        "char":  26,
+        "comment": "Missing space between options",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "hello {|4.2| :number min=|a|max=3}",
+        "char": 28,
+        "comment": "Missing whitespace between valid options",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "hello {|4.2| :number min=|\\a|}",
+        "char": 27,
+        "comment": "Ill-formed RHS of option -- the error should be within the RHS, not after parsing options",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "no-equal {|42| :number   {}",
+        "char": 25,
+        "comment": "Junk after annotation",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "bad {:placeholder option=}",
+        "char": 25,
+        "comment": "Missing RHS of option",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "bad {:placeholder option}",
+        "char": 24,
+        "comment": "Missing RHS of option",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "bad {$placeholder option}",
+        "char": 18,
+        "comment": "Annotation is not a function or reserved text",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "no {$placeholder end",
+        "char": 17,
+        "comment": "Annotation is not a function or reserved text",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match  * {{foo}}",
+        "char": 8,
+        "comment": "Missing expression in selectors",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match |x|  * {{foo}}",
+        "char": 7,
+        "comment": "Non-expression in selectors",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".match {|x|}  * foo",
+        "char": 19,
+        "comment": "Missing RHS in variant",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{$:abc}",
+        "char": 2,
+        "comment": "Variable names can't start with a : or -",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{$-abc}",
+        "char": 2,
+        "comment": "Variable names can't start with a : or -",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{$bar+foo}",
+        "char": 5,
+        "comment": "Missing space before annotation. Note that {{$bar:foo}} and {{$bar-foo}} are valid, because variable names can contain a ':' or a '-'",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{|3.14|:foo}",
+        "char": 7,
+        "comment": "Missing space before annotation.",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{|3.14|-foo}",
+        "char": 7,
+        "comment": "Missing space before annotation.",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": "{|3.14|+foo}",
+        "char": 7,
+        "comment": "Missing space before annotation.",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".local $foo = {$bar} .match {$foo}  :one {one} * {other}",
+        "char": 36,
+        "comment": "Unquoted literals can't begin with a ':'",
+        "errors": [{"type": "parse-error"}]
+    },
+    {
+        "src": ".local $foo = {$bar :fun option=:a} {{bar {$foo}}}",
+        "char": 32,
+        "comment": "Unquoted literals can't begin with a ':'",
+        "errors": [{"type": "parse-error"}]
+    },
+    { "src": "{|foo| {#markup}}", "char": 7, "comment": "Markup in wrong place", "errors": [{"type": "parse-error"}] },
+    { "src": "{|foo| #markup}", "char": 7, "comment": "Markup in wrong place", "errors": [{"type": "parse-error"}] },
+    { "src": "{|foo| {#markup/}}", "char": 7, "comment": "Markup in wrong place", "errors": [{"type": "parse-error"}] },
+    { "src": "{|foo| {/markup}}", "char": 7, "comment": "Markup in wrong place", "errors": [{"type": "parse-error"}] },
+    { "src": ".input $x = {|1|} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression", "errors": [{"type": "parse-error"}] },
+    { "src": ".input $x = {:number} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression", "errors": [{"type": "parse-error"}] },
+    { "src": ".input {|1| :number} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression", "errors": [{"type": "parse-error"}] },
+    { "src": ".input {:number} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression", "errors": [{"type": "parse-error"}] },
+    { "src": ".input {|1|} {{{$x}}}", "char": 7, "comment": ".input with non-variable-expression", "errors": [{"type": "parse-error"}] },
+    { "src": ".", "char": 1, "errors": [{"type": "parse-error"}]},
+    { "src":  "{", "char": 1, "errors": [{"type": "parse-error"}]},
+    { "src":  "}", "char": 0, "errors": [{"type": "parse-error"}]},
+    { "src":  "{}", "char": 1, "errors": [{"type": "parse-error"}]},
+    { "src":  "{{", "char": 2, "errors": [{"type": "parse-error"}]},
+    { "src":  "{{}", "char": 3, "errors": [{"type": "parse-error"}]},
+    { "src":  "{{}}}", "char": 4, "errors": [{"type": "parse-error"}]},
+    { "src":  "{|foo| #markup}", "char": 7, "errors": [{"type": "parse-error"}]},
+    { "src":  "{{missing end brace}", "char": 20, "errors": [{"type": "parse-error"}]},
+    { "src":  "{{missing end braces", "char": 20, "errors": [{"type": "parse-error"}]},
+    { "src":  "{{missing end {$braces", "char": 22, "errors": [{"type": "parse-error"}]},
+    { "src":  "{{extra}} content", "char": 9, "errors": [{"type": "parse-error"}]},
+    { "src":  "empty { } placeholder", "char": 8, "errors": [{"type": "parse-error"}]},
+    { "src":  "missing space {42:func}", "char": 17, "errors": [{"type": "parse-error"}]},
+    { "src":  "missing space {|foo|:func}", "char": 20, "errors": [{"type": "parse-error"}]},
+    { "src":  "missing space {|foo|@bar}", "char": 20, "errors": [{"type": "parse-error"}]},
+    { "src":  "missing space {:func@bar}", "char": 20, "errors": [{"type": "parse-error"}]},
+    { "src":  "{:func @bar@baz}", "char": 11, "errors": [{"type": "parse-error"}]},
+    { "src":  "{:func @bar=42@baz}", "char": 14, "errors": [{"type": "parse-error"}]},
+    { "src":  "{+reserved@bar}", "char": 10, "errors": [{"type": "parse-error"}]},
+    { "src":  "{&private@bar}", "char": 9, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:} placeholder", "char": 6, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {\\u0000placeholder}", "char": 5, "errors": [{"type": "parse-error"}]},
+    { "src":  "no-equal {|42| :number minimumFractionDigits 2}", "char": 45, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder option=}", "char": 25, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder option value}", "char": 25, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder option:value}", "char": 30, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder option}", "char": 24, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder:}", "char": 18, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {::placeholder}", "char": 6, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder::foo}", "char": 18, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder option:=x}", "char": 25, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder :option=x}", "char": 18, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder option::x=y}", "char": 25, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {$placeholder option}", "char": 18, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder @attribute=}", "char": 29, "errors": [{"type": "parse-error"}]},
+    { "src":  "bad {:placeholder @attribute=@foo}", "char": 29, "errors": [{"type": "parse-error"}]},
+    { "src":  "no {placeholder end", "char": 16, "errors": [{"type": "parse-error"}]},
+    { "src":  "no {$placeholder end", "char": 17, "errors": [{"type": "parse-error"}]},
+    { "src":  "no {:placeholder end", "char": 20, "errors": [{"type": "parse-error"}]},
+    { "src":  "no {|placeholder| end", "char": 18, "errors": [{"type": "parse-error"}]},
+    { "src":  "no {|literal} end", "char": 17, "errors": [{"type": "parse-error"}]},
+    { "src":  "no {|literal or placeholder end", "char": 31, "errors": [{"type": "parse-error"}]},
+    { "src":  ".local bar = {|foo|} {{_}}", "char": 7, "errors": [{"type": "parse-error"}]},
+    { "src":  ".local #bar = {|foo|} {{_}}", "char": 7, "errors": [{"type": "parse-error"}]},
+    { "src":  ".local $bar {|foo|} {{_}}", "char": 12, "errors": [{"type": "parse-error"}]},
+    { "src":  ".local $bar = |foo| {{_}}", "char": 14, "errors": [{"type": "parse-error"}]},
+    { "src":  ".match {#foo} * {{foo}}", "char": 8, "errors": [{"type": "parse-error"}]},
+    { "src":  ".match {} * {{foo}}", "char": 8, "errors": [{"type": "parse-error"}]},
+    { "src":  ".match {|foo| :x} {|bar| :x} ** {{foo}}", "char": 30, "errors": [{"type": "parse-error"}]},
+    { "src":  ".match * {{foo}}", "char": 7, "errors": [{"type": "parse-error"}]},
+    { "src":  ".match {|x| :x} * foo", "char": 21, "errors": [{"type": "parse-error"}]},
+    { "src":  ".match {|x| :x} * {{foo}} extra", "char": 31, "errors": [{"type": "parse-error"}]},
+    { "src":  ".match |x| * {{foo}}", "char": 7, "errors": [{"type": "parse-error"}]},
+    { "src": ".match {|foo| :string} o:ne {{one}}  * {{other}}", "char": 24, "comment" : "tests for ':' in unquoted literals (not allowed)" , "errors": [{"type": "parse-error"}]},
+    { "src": ".match {|foo| :string} one: {{one}}  * {{other}}", "char": 26, "comment" : "tests for ':' in unquoted literals (not allowed)" , "errors": [{"type": "parse-error"}]},
+    { "src": ".local $foo = {|42| :number option=a:b} {{bar {$foo}}}", "char": 36, "comment" : "tests for ':' in unquoted literals (not allowed)" , "errors": [{"type": "parse-error"}]},
+    { "src": ".local $foo = {|42| :number option=a:b:c} {{bar {$foo}}}", "char": 36, "comment" : "tests for ':' in unquoted literals (not allowed)" , "errors": [{"type": "parse-error"}]},
+    { "src": "{$bar:foo}", "char": 5, "comment" : "tests for ':' in unquoted literals (not allowed)", "errors": [{"type": "parse-error"}]},
+    {
+        "src":  ".match {1} {{_}}",
+        "char": 12,
+        "comment": "Disambiguating a wrong .match from an unsupported statement",
+        "errors": [{"type": "parse-error"}]
+    }
+]
diff --git a/icu4c/source/test/testdata/message2/syntax-errors-end-of-input.json b/testdata/message2/syntax-errors-end-of-input.json
similarity index 100%
rename from icu4c/source/test/testdata/message2/syntax-errors-end-of-input.json
rename to testdata/message2/syntax-errors-end-of-input.json
diff --git a/icu4c/source/test/testdata/message2/tricky-declarations.json b/testdata/message2/tricky-declarations.json
similarity index 100%
rename from icu4c/source/test/testdata/message2/tricky-declarations.json
rename to testdata/message2/tricky-declarations.json
diff --git a/icu4c/source/test/testdata/message2/valid-tests.json b/testdata/message2/valid-tests.json
similarity index 68%
rename from icu4c/source/test/testdata/message2/valid-tests.json
rename to testdata/message2/valid-tests.json
index c0047a2c3c2e..f037c49d4b48 100644
--- a/icu4c/source/test/testdata/message2/valid-tests.json
+++ b/testdata/message2/valid-tests.json
@@ -1,8 +1,8 @@
 [
     { "src": "hello {|4.2| :number}", "exp": "hello 4.2"},
     { "src": "hello {|4.2| :number minimumFractionDigits=2}", "exp": "hello 4.20"},
-    { "src": "hello {|4.2| :number minimumFractionDigits = 2}", "exp": "hello 4.20"},
-    { "src": "hello {|4.2| :number minimumFractionDigits= 2}", "exp": "hello 4.20"},
+    { "src": "hello {|4.2| :number minimumFractionDigits = 2}", "exp": "hello 4.20" },
+    { "src": "hello {|4.2| :number minimumFractionDigits= 2}", "exp": "hello 4.20" },
     { "src": "hello {|4.2| :number minimumFractionDigits =2}", "exp": "hello 4.20"},
     { "src": "hello {|4.2| :number minimumFractionDigits=2  }", "exp": "hello 4.20"},
     { "src": "hello {|4.2| :number minimumFractionDigits=2 bar=3}", "exp": "hello 4.20"},
@@ -46,34 +46,60 @@
     { "src": "{-1}", "exp": "-1"},
     { "src": "{0}", "exp": "0"},
     { "src": "{0.0123}", "exp": "0.0123"},
-    { "src": "{1.234e5}", "exp": "1.234e5"},
-    { "src": "{1.234E5}", "exp": "1.234E5"},
-    { "src": "{1.234E+5}", "exp": "1.234E+5"},
-    { "src": "{1.234e-5}", "exp": "1.234e-5"},
-    { "src": "{42e5}", "exp": "42e5"},
-    { "src": "{42e0}", "exp": "42e0"},
-    { "src": "{42e000}", "exp": "42e000"},
-    { "src": "{42e369}", "exp": "42e369"},
+    { "src": "{1.234e5}", "exp": "1.234e5",
+      "ignoreJava": "See ICU-22810"},
+    { "src": "{1.234E5}", "exp": "1.234E5",
+      "ignoreJava": "See ICU-22810"},
+    { "src": "{1.234E+5}", "exp": "1.234E+5",
+      "ignoreJava": "See ICU-22810"},
+    { "src": "{1.234e-5}", "exp": "1.234e-5",
+      "ignoreJava": "See ICU-22810"},
+    { "src": "{42e5}", "exp": "42e5",
+      "ignoreJava": "See ICU-22810"},
+    { "src": "{42e0}", "exp": "42e0",
+      "ignoreJava": "See ICU-22810"},
+    { "src": "{42e000}", "exp": "42e000",
+      "ignoreJava": "See ICU-22810"},
+    { "src": "{42e369}", "exp": "42e369",
+      "ignoreJava": "See ICU-22810"},
     { "src": "hello {|3| :number   }", "exp": "hello 3" },
-    { "src": "{:foo}", "errors": [{ "type": "missing-func" }] },
-    { "src": "{:foo }", "errors": [{ "type": "missing-func" }] },
-    { "src": "{:foo   }", "errors": [{ "type": "missing-func" }] },
-    { "src": "{:foo k=v}", "errors": [{ "type": "missing-func" }] },
-    { "src": "{:foo k=v   }", "errors": [{ "type": "missing-func" }] },
-    { "src": "{:foo k1=v1   k2=v2}", "errors": [{ "type": "missing-func" }] },
-    { "src": "{:foo k1=v1   k2=v2   }", "errors": [{ "type": "missing-func" }] },
+    { "src": "{:foo}", "errors": [{ "type": "missing-func" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{:foo }", "errors": [{ "type": "missing-func" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{:foo   }", "errors": [{ "type": "missing-func" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{:foo k=v}", "errors": [{ "type": "missing-func" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{:foo k=v   }", "errors": [{ "type": "missing-func" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{:foo k1=v1   k2=v2}", "errors": [{ "type": "missing-func" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{:foo k1=v1   k2=v2   }", "errors": [{ "type": "missing-func" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
     { "src": "{|3.14| }", "exp": "3.14" },
     { "src": "{|3.14|    }", "exp": "3.14" },
     { "src": "{|3.14|    :number}", "exp": "3.14" },
     { "src": "{|3.14|    :number   }", "exp": "3.14" },
-    { "src": "{$bar }", "errors": [{ "type": "unresolved-var" }] },
-    { "src": "{$bar    }", "errors": [{ "type": "unresolved-var" }] },
-    { "src": "{$bar    :foo}", "errors": [{ "type": "unresolved-var" }] },
-    { "src": "{$bar    :foo   }", "errors": [{ "type": "unresolved-var" }] },
-    { "src": "{$bar-foo}", "errors": [{ "type": "unresolved-var" }] },
+    { "src": "{$bar }", "errors": [{ "type": "unresolved-var" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{$bar    }", "errors": [{ "type": "unresolved-var" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{$bar    :foo}", "errors": [{ "type": "unresolved-var" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{$bar    :foo   }", "errors": [{ "type": "unresolved-var" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
+    { "src": "{$bar-foo}", "errors": [{ "type": "unresolved-var" }],
+      "ignoreJava": "See https://github.com/unicode-org/message-format-wg/issues/782" },
     { "src": ".local $foo = {|hello|} .local $foo = {$foo} {{{$foo}}}",
       "errors": [{ "type": "duplicate-declaration" }] },
     { "src": "good {placeholder}", "exp": "good placeholder" },
+    {
+        "src": "a\\\\qbc",
+        "exp": "a\\qbc",
+        "comment": "pattern -> escaped-char -> backslash backslash"
+    },
+
     {
         "src": "{$one} and {$two}",
         "params": { "one": "1.3", "two": "4.2" },