Fixes #15127: Added optional error code to flag missing return type and tests

mypy/errorcodes.py

@@ -319,5 +319,10 @@ def __hash__(self) -> int:
     default_enabled=False,
 )
 
+MISSING_RETURN_ANNOTATION = ErrorCode(
+    "missing-return-annotation", "Function has no return type annotation", "Optional"
+)
+
+
 # This copy will not include any error codes defined later in the plugins.
 mypy_error_codes = error_codes.copy()

mypy/semanal.py

@@ -57,7 +57,7 @@
 
 from mypy import errorcodes as codes, message_registry
 from mypy.constant_fold import constant_fold_expr
-from mypy.errorcodes import PROPERTY_DECORATOR, ErrorCode
+from mypy.errorcodes import MISSING_RETURN_ANNOTATION, PROPERTY_DECORATOR, ErrorCode
 from mypy.errors import Errors, report_internal_error
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
 from mypy.message_registry import ErrorMessage
@@ -926,6 +926,24 @@ def visit_func_def(self, defn: FuncDef) -> None:
             with self.inside_except_star_block_set(value=False):
                 self.analyze_func_def(defn)
 
+                # Missing return annotation check (optional error code)
+                from mypy.types import CallableType
+
+                if (
+                    # Makes sure return type is not specified, ignores lambdas and special cases
+                    isinstance(defn.type, CallableType)
+                    and defn.type.ret_type is None
+                    and any(arg.type_annotation is not None for arg in defn.arguments)
+                    and defn.name != "__init__"
+                    and not defn.is_lambda
+                    and not defn.is_overload
+                ):
+                    self.msg.fail(
+                        f'Function "{defn.name}" has no return type annotation',
+                        defn,
+                        code=MISSING_RETURN_ANNOTATION,
+                    )
+
     def function_fullname(self, fullname: str) -> str:
         if self.current_overload_item is None:
             return fullname

test-data/unit/check-missing-return-annotation.test

@@ -0,0 +1,8 @@
+[case test_missing_return_annotation_ok]
+# flags: --enable-error-code missing-return-annotation
+def g(x: int) -> int:
+    return x
+
+[case test_missing_return_annotation_off_by_default]
+def h(x: int):
+    return x  # no error expected, since code is opt-in