From f06e909d34215e353c53d5735915887e1395a5a4 Mon Sep 17 00:00:00 2001
From: Gleb Belov <gleb@ampl.com>
Date: Thu, 27 Jul 2023 16:04:58 +1000
Subject: [PATCH] #218 Task 2: Hint to use 'writesol'

---
 include/mp/backend-std.h         | 24 ++++++++++++++++++++++--
 solvers/gurobi/CHANGES.gurobi.md |  4 ++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/include/mp/backend-std.h b/include/mp/backend-std.h
index 752e43b6a..513482d5e 100644
--- a/include/mp/backend-std.h
+++ b/include/mp/backend-std.h
@@ -211,7 +211,7 @@ class StdBackend :
 
     SetupTimerAndInterrupter();
     if (exportFileMode() > 0)
-      DoWriteProblem(export_file_name());
+      ExportModel(export_file_name());
 
     // exportFileMode == 2 -> do not solve, just export
     if (exportFileMode() != 2) 
@@ -766,7 +766,10 @@ class StdBackend :
     return 0;
   }
   std::string export_file_name() const {
-    std::string name = storedOptions_.export_file_.empty() ? storedOptions_.just_export_file_ : storedOptions_.export_file_;
+    std::string name
+        = storedOptions_.export_file_.empty()
+        ? storedOptions_.just_export_file_
+        : storedOptions_.export_file_;
     if (((name.front() == '"') && (name.back() == '"')) ||
       ((name.front() == '\'') && (name.back() == '\'')))
       return name.substr(1, name.length() - 2);
@@ -944,6 +947,23 @@ class StdBackend :
         DoWriteSolution(storedOptions_.export_sol_);
   }
 
+  /// Write model
+  virtual void ExportModel(const std::string& filename) {
+    try {
+      DoWriteProblem(filename);
+    } catch (const std::exception& exc) {
+      auto msg
+          = std::string("Model export failed:\n")
+          + exc.what();
+      if (IMPL_HAS_STD_FEATURE(WRITE_SOLUTION))
+        msg +=
+            "\n    Note: to export solutions and results\n"
+            "    in the solver's native formats,\n"
+            "    use option 'tech:writesolution'";
+      MP_RAISE(msg);
+    }
+  }
+
   /// Virtual destructor
   virtual ~StdBackend() { }
 };
diff --git a/solvers/gurobi/CHANGES.gurobi.md b/solvers/gurobi/CHANGES.gurobi.md
index d215c0c29..0704bb4bf 100644
--- a/solvers/gurobi/CHANGES.gurobi.md
+++ b/solvers/gurobi/CHANGES.gurobi.md
@@ -1,6 +1,10 @@
 Summary of recent updates to gurobi for AMPL
 ============================================
 
+## unreleased
+- Hint when 'writeprob' fails: use 'writesol'
+
+
 ## 20230726
 - Fixed inequalities of integer expressions with
   non-integer constants, see test_int_non_int.mod.