Adding optional error flag missing return type

Adding optional error flag for missing return type, when a function has at least one typed argument. This will address issue python#15127

Co-Authored-By: Leonardo Abreu Santos <31524775+saintleonar@users.noreply.github.com>

mypy/checker.py

@@ -1420,8 +1420,15 @@ def is_unannotated_any(t: Type) -> bool:
             not is_unannotated_any(t) for t in fdef.type.arg_types + [fdef.type.ret_type]
         )
 
+        has_explicit_arg_annotation = isinstance(fdef.type, CallableType) and any(
+            not is_unannotated_any(t) for t in fdef.type.arg_types
+        )
+
         show_untyped = not self.is_typeshed_stub or self.options.warn_incomplete_stub
         check_incomplete_defs = self.options.disallow_incomplete_defs and has_explicit_annotation
+        check_incomplete_defs_arg = (
+            self.options.disallow_incomplete_defs and has_explicit_arg_annotation
+        )
         if show_untyped and (self.options.disallow_untyped_defs or check_incomplete_defs):
             if fdef.type is None and self.options.disallow_untyped_defs:
                 if not fdef.arguments or (
@@ -1439,7 +1446,9 @@ def is_unannotated_any(t: Type) -> bool:
                     self.fail(message_registry.FUNCTION_TYPE_EXPECTED, fdef)
             elif isinstance(fdef.type, CallableType):
                 ret_type = get_proper_type(fdef.type.ret_type)
-                if is_unannotated_any(ret_type):
+                if check_incomplete_defs_arg and is_unannotated_any(ret_type):
+                    self.fail(message_registry.RETURN_TYPE_EXPECTED_TYPED_ARG, fdef)
+                elif is_unannotated_any(ret_type):
                     self.fail(message_registry.RETURN_TYPE_EXPECTED, fdef)
                 elif fdef.is_generator:
                     if is_unannotated_any(

mypy/errorcodes.py

@@ -151,6 +151,11 @@ def __hash__(self) -> int:
 NO_UNTYPED_DEF: Final[ErrorCode] = ErrorCode(
     "no-untyped-def", "Check that every function has an annotation", "General"
 )
+NO_UNTYPED_RET: Final[ErrorCode] = ErrorCode(
+    "no-untyped-ret",
+    "Check that every function with at least one typed argument has a typed return",
+    "General",
+)
 NO_UNTYPED_CALL: Final = ErrorCode(
     "no-untyped-call",
     "Disallow calling functions without type annotations from annotated functions",

mypy/message_registry.py

@@ -119,6 +119,9 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 RETURN_TYPE_EXPECTED: Final = ErrorMessage(
     "Function is missing a return type annotation", codes.NO_UNTYPED_DEF
 )
+RETURN_TYPE_EXPECTED_TYPED_ARG: Final = ErrorMessage(
+    "Function is missing a return type annotation", codes.NO_UNTYPED_RET
+)
 ARGUMENT_TYPE_EXPECTED: Final = ErrorMessage(
     "Function is missing a type annotation for one or more arguments", codes.NO_UNTYPED_DEF
 )