Consider overlap between instances and callables (#17389)

Fixes #8869

The fix seems straightforward.

mypy/meet.py

@@ -7,6 +7,7 @@
 from mypy.maptype import map_instance_to_supertype
 from mypy.state import state
 from mypy.subtypes import (
+    find_member,
     is_callable_compatible,
     is_equivalent,
     is_proper_subtype,
@@ -477,9 +478,22 @@ def _type_object_overlap(left: Type, right: Type) -> bool:
             ignore_pos_arg_names=True,
             allow_partial_overlap=True,
         )
-    elif isinstance(left, CallableType):
+
+    call = None
+    other = None
+    if isinstance(left, CallableType) and isinstance(right, Instance):
+        call = find_member("__call__", right, right, is_operator=True)
+        other = left
+    if isinstance(right, CallableType) and isinstance(left, Instance):
+        call = find_member("__call__", left, left, is_operator=True)
+        other = right
+    if isinstance(get_proper_type(call), FunctionLike):
+        assert call is not None and other is not None
+        return _is_overlapping_types(call, other)
+
+    if isinstance(left, CallableType):
         left = left.fallback
-    elif isinstance(right, CallableType):
+    if isinstance(right, CallableType):
         right = right.fallback
 
     if isinstance(left, LiteralType) and isinstance(right, LiteralType):

test-data/unit/check-statements.test

@@ -2307,3 +2307,38 @@ class Outer:
         class Inner:
             break  # E: "break" outside loop
 [builtins fixtures/list.pyi]
+
+[case testCallableInstanceOverlapAllowed]
+# flags: --warn-unreachable
+from typing import Any, Callable, List
+
+class CAny:
+    def __call__(self) -> Any: ...
+class CNone:
+    def __call__(self) -> None: ...
+class CWrong:
+    def __call__(self, x: int) -> None: ...
+
+def describe(func: Callable[[], None]) -> str:
+    if isinstance(func, CAny):
+        return "CAny"
+    elif isinstance(func, CNone):
+        return "CNone"
+    elif isinstance(func, CWrong):
+        return "CWrong"  # E: Statement is unreachable
+    else:
+        return "other"
+
+class C(CAny):
+    def __call__(self) -> None: ...
+
+def f():
+    pass
+
+describe(CAny())
+describe(C())
+describe(CNone())
+describe(CWrong())  # E: Argument 1 to "describe" has incompatible type "CWrong"; expected "Callable[[], None]" \
+                    # N: "CWrong.__call__" has type "Callable[[Arg(int, 'x')], None]"
+describe(f)
+[builtins fixtures/isinstancelist.pyi]