ICU-22843 UnicodeString <-> std::u16string_view / wstring_view via te…

…mplates

icu4c/source/common/unicode/char16ptr.h

@@ -12,6 +12,7 @@
 #if U_SHOW_CPLUSPLUS_API
 
 #include <cstddef>
+#include <string_view>
 
 /**
  * \file
@@ -306,6 +307,36 @@ inline OldUChar *toOldUCharPtr(char16_t *p) {
     return reinterpret_cast<OldUChar *>(p);
 }
 
+#ifndef U_FORCE_HIDE_INTERNAL_API
+/**
+ * Is T convertible to a std::u16string_view or to a 16-bit std::wstring_view?
+ * @internal
+ */
+template<typename T>
+constexpr bool ConvertibleToU16StringView =
+    std::is_convertible_v<T, std::u16string_view> ||
+    (U_SIZEOF_WCHAR_T==2 && std::is_convertible_v<T, std::wstring_view>);
+
+namespace internal {
+/**
+ * Pass-through overload.
+ * @internal
+ */
+inline std::u16string_view toU16StringView(std::u16string_view sv) { return sv; }
+
+#if U_SIZEOF_WCHAR_T==2
+/**
+ * Basically undefined behavior but sometimes necessary conversion
+ * from std::wstring_view to std::u16string_view.
+ * @internal
+ */
+inline std::u16string_view toU16StringView(std::wstring_view sv) {
+    return { ConstChar16Ptr(sv.data()), sv.length() };
+}
+#endif
+}  // internal
+#endif  // U_FORCE_HIDE_INTERNAL_API
+
 U_NAMESPACE_END
 
 #endif /* U_SHOW_CPLUSPLUS_API */

icu4c/source/common/unicode/platform.h

@@ -735,7 +735,9 @@
  * @{
  * \def U_DECLARE_UTF16
  * Do not use this macro because it is not defined on all platforms.
- * Use the UNICODE_STRING or U_STRING_DECL macros instead.
+ * In C++, use std::u16string_view literals, see the UNICODE_STRING docs.
+ * In C, use u"UTF-16 literals".
+ * See also the public U_STRING_DECL macro.
  * @internal
  */
 #ifdef U_DECLARE_UTF16