Add migration test for compatibility

test/integration/test_migrations.py

@@ -151,3 +151,29 @@ def test_back_default_compatibility_migration():
     with patch('conan.api.conan_api.ClientMigrator', new=lambda *args, **kwargs: migrator):
         t.run("-v")  # Fire the backward migration
         assert f"WARN: Downgrading cache from Conan {conan_version} to 2.3.2" in t.out
+
+def test_migration_cppstd_compat_different_file():
+    t = TestClient()
+    t.run("-v")
+    assert 'WARN: The "cppstd_compat.py" file is deprecated' not in t.out
+
+    version_txt_file_path = os.path.join(t.cache_folder, "version.txt")
+    save(version_txt_file_path, "2.11")
+
+    cppstd_compat_file = os.path.join(t.cache_folder, "extensions", "plugins", "compatibility", "cppstd_compat.py")
+    save(cppstd_compat_file, "custom file content")
+
+    t.run("-v")
+    assert 'WARN: The "cppstd_compat.py" file is deprecated' in t.out
+    # Exists because it was not the default one
+    assert os.path.exists(cppstd_compat_file)
+
+    save(version_txt_file_path, "2.11")
+    cppstd_compat_file = os.path.join(t.cache_folder, "extensions", "plugins", "compatibility",
+                                      "cppstd_compat.py")
+    save(cppstd_compat_file, "# This file was generated by Conan. Remove this comment if you edit this file or Conan")
+
+    t.run("-v")
+    assert 'Migration: Successfully removed cppstd_compat.py' in t.out
+    # Removed because it was the default
+    assert not os.path.exists(cppstd_compat_file)