Multiple inheritance considers callable objects as subtypes of functions (#14855)

Let the multiple inheritance checks consider callable objects as
possible subtypes of usual functions (Fixes #14852).

The solution is inspired by the `visit_instance` method of
`SubtypeVisitor`. The
`testMultipleInheritanceOverridingOfFunctionsWithCallableInstances`
tests the new functionality for decorated and non-decorated functions
and callable objects.

Author: tyralla

Diff for: mypy/checker.py

@@ -2485,7 +2485,14 @@ class C(B, A[int]): ...  # this is unsafe because...
         first_type = get_proper_type(self.determine_type_of_member(first))
         second_type = get_proper_type(self.determine_type_of_member(second))
 
-        if isinstance(first_type, FunctionLike) and isinstance(second_type, FunctionLike):
+        # start with the special case that Instance can be a subtype of FunctionLike
+        call = None
+        if isinstance(first_type, Instance):
+            call = find_member("__call__", first_type, first_type, is_operator=True)
+        if call and isinstance(second_type, FunctionLike):
+            second_sig = self.bind_and_map_method(second, second_type, ctx, base2)
+            ok = is_subtype(call, second_sig, ignore_pos_arg_names=True)
+        elif isinstance(first_type, FunctionLike) and isinstance(second_type, FunctionLike):
             if first_type.is_type_obj() and second_type.is_type_obj():
                 # For class objects only check the subtype relationship of the classes,
                 # since we allow incompatible overrides of '__init__'/'__new__'

Diff for: test-data/unit/check-multiple-inheritance.test

@@ -668,3 +668,41 @@ class D1(B[str], C1): ...
 class D2(B[Union[int, str]], C2): ...
 class D3(C2, B[str]): ...
 class D4(B[str], C2): ...  # E: Definition of "foo" in base class "A" is incompatible with definition in base class "C2"
+
+
+[case testMultipleInheritanceOverridingOfFunctionsWithCallableInstances]
+from typing import Any, Callable
+
+def dec1(f: Callable[[Any, int], None]) -> Callable[[Any, int], None]: ...
+
+class F:
+    def __call__(self, x: int) -> None: ...
+
+def dec2(f: Callable[[Any, int], None]) -> F: ...
+
+class B1:
+    def f(self, x: int) -> None: ...
+
+class B2:
+    @dec1
+    def f(self, x: int) -> None: ...
+
+class B3:
+    @dec2
+    def f(self, x: int) -> None: ...
+
+class B4:
+    f = F()
+
+class C12(B1, B2): ...
+class C13(B1, B3): ...  # E: Definition of "f" in base class "B1" is incompatible with definition in base class "B3"
+class C14(B1, B4): ...  # E: Definition of "f" in base class "B1" is incompatible with definition in base class "B4"
+class C21(B2, B1): ...
+class C23(B2, B3): ...  # E: Definition of "f" in base class "B2" is incompatible with definition in base class "B3"
+class C24(B2, B4): ...  # E: Definition of "f" in base class "B2" is incompatible with definition in base class "B4"
+class C31(B3, B1): ...
+class C32(B3, B2): ...
+class C34(B3, B4): ...
+class C41(B4, B1): ...
+class C42(B4, B2): ...
+class C43(B4, B3): ...