Fix crash with overload and callable object decorators

Fixes the crash in #8356, as identified by @pranavrajpal

Note that we still don't support the singledispatch in that issue, since
we get 'overloaded function has no attribute "register"', but that's
much easier to work around than a crash.

mypy/checker.py

@@ -507,8 +507,14 @@ def check_overlapping_overloads(self, defn: OverloadedFuncDef) -> None:
             # decorator or if the implementation is untyped -- we gave up on the types.
             inner_type = get_proper_type(inner_type)
             if inner_type is not None and not isinstance(inner_type, AnyType):
-                assert isinstance(inner_type, CallableType)
-                impl_type = inner_type
+                if isinstance(inner_type, CallableType):
+                    impl_type = inner_type
+                elif isinstance(inner_type, Instance):
+                    inner_call = find_member('__call__', inner_type, inner_type, is_operator=True)
+                    assert inner_call is not None
+                    impl_type = cast(CallableType, inner_call)
+                else:
+                    assert False
 
         is_descriptor_get = defn.info and defn.name == "__get__"
         for i, item in enumerate(defn.items):

test-data/unit/check-overloading.test

@@ -5339,3 +5339,47 @@ def register(cls: Any) -> Any: return None
 x = register(Foo)
 reveal_type(x)  # N: Revealed type is "builtins.int"
 [builtins fixtures/dict.pyi]
+
+
+[case testOverloadWithObjectDecorator]
+from typing import Any, Callable, Union, overload
+
+class A:
+    def __call__(self, *arg, **kwargs) -> None: ...
+
+def dec_a(f: Callable[..., Any]) -> A:
+    return A()
+
+@overload
+def f_a(arg: int) -> None: ...
+@overload
+def f_a(arg: str) -> None: ...
+@dec_a
+def f_a(arg): ...
+
+class B:
+    def __call__(self, arg: Union[int, str]) -> None: ...
+
+def dec_b(f: Callable[..., Any]) -> B:
+    return B()
+
+@overload
+def f_b(arg: int) -> None: ...
+@overload
+def f_b(arg: str) -> None: ...
+@dec_b
+def f_b(arg): ...
+
+class C:
+    def __call__(self, arg: int) -> None: ...
+
+def dec_c(f: Callable[..., Any]) -> C:
+    return C()
+
+@overload
+def f_c(arg: int) -> None: ...
+@overload
+def f_c(arg: str) -> None: ...
+@dec_c  # E: Overloaded function implementation does not accept all possible arguments of signature 2
+def f_c(arg): ...
+[builtins fixtures/dict.pyi]