test: cover compatibility function arithmetic returns

tests/test_validate_typecheck_diagnostics.py

@@ -202,6 +202,12 @@ class ValidateTypecheckDiagnosticsTests(unittest.TestCase):
             MODULE.EXPECTED,
         )
 
+    def test_includes_compatibility_function_arithmetic_return_success_fixture(self):
+        self.assertIn(
+            "tests/typecheck_function_compat_return_addition_good.claro",
+            MODULE.EXPECTED_OK,
+        )
+
     def test_includes_lowercase_compatibility_method_return_success_fixture(self):
         self.assertIn(
             "tests/typecheck_method_compat_lowercase_return_good.claro",

tests/typecheck_function_compat_return_addition_good.claro

@@ -0,0 +1,6 @@
+TEACH total TAKES amount RETURNS NUMBER
+    CHECK TYPE amount IS NUMBER
+    RETURN amount + 1
+LEARNED
+
+CALL total WITH 4