Let overload item have a wider return type than implementation

A wider return type can be useful if a decorator used for the overload
implementation gets a more precise return type as part of a typeshed
update.

Closes #12434.

mypy/checker.py

@@ -600,7 +600,8 @@ def check_overlapping_overloads(self, defn: OverloadedFuncDef) -> None:
                     self.msg.overloaded_signatures_arg_specific(i + 1, defn.impl)
 
                 # Is the overload alternative's return type a subtype of the implementation's?
-                if not is_subtype_no_promote(sig1.ret_type, impl.ret_type):
+                if not (is_subtype_no_promote(sig1.ret_type, impl.ret_type) or
+                        is_subtype_no_promote(impl.ret_type, sig1.ret_type)):
                     self.msg.overloaded_signatures_ret_specific(i + 1, defn.impl)
 
     # Here's the scoop about generators and coroutines.

test-data/unit/check-overloading.test

@@ -6328,3 +6328,15 @@ if True:
 def f2(x): ...
 if True:
     def f2(x): ...  # E: Name "f2" already defined on line 17
+
+[case testOverloadItemHasMoreGeneralReturnType]
+from typing import overload
+
+@overload
+def f() -> object: ...
+
+@overload
+def f(x: int) -> object: ...
+
+def f(x: int = 0) -> int:
+    return x