test: cover compatibility method subtraction diagnostics

tests/test_validate_typecheck_diagnostics.py

@@ -128,6 +128,12 @@ class ValidateTypecheckDiagnosticsTests(unittest.TestCase):
             MODULE.EXPECTED,
         )
 
+    def test_includes_compatibility_method_subtraction_return_diagnostic_fixture(self):
+        self.assertIn(
+            "tests/typecheck_method_compat_return_subtraction_bad.claro",
+            MODULE.EXPECTED,
+        )
+
     def test_includes_lowercase_compatibility_method_return_success_fixture(self):
         self.assertIn(
             "tests/typecheck_method_compat_lowercase_return_good.claro",

tests/typecheck_method_compat_return_subtraction_bad.claro

@@ -0,0 +1,6 @@
+CLASS Player
+    TEACH total TAKES amount RETURNS NUMBER
+        CHECK TYPE amount IS NUMBER
+        RETURN amount - "oops"
+    LEARNED
+ENDCLASS