test: cover compatibility branch returns

tests/test_validate_typecheck_diagnostics.py

@@ -111,6 +111,12 @@ class ValidateTypecheckDiagnosticsTests(unittest.TestCase):
     def test_includes_branch_missing_function_return_fixture(self):
         self.assertIn("tests/typecheck_function_branch_missing_return_bad.claro", MODULE.EXPECTED)
 
+    def test_includes_compatibility_branch_missing_function_return_fixture(self):
+        self.assertIn(
+            "tests/typecheck_function_compat_branch_missing_return_bad.claro",
+            MODULE.EXPECTED,
+        )
+
     def test_includes_empty_function_return_fixture(self):
         self.assertIn("tests/typecheck_function_empty_return_bad.claro", MODULE.EXPECTED)
 

tests/typecheck_function_compat_branch_missing_return_bad.claro

@@ -0,0 +1,5 @@
+TEACH choose TAKES flag RETURNS NUMBER
+    IF flag
+        RETURN 1
+    END
+LEARNED