Fix is_overlapping_types for generic callables (python#21208)

Fixes python#21182

This issue exposed this pre-existing deficiency in is_overlapping_types

Author: hauntsaninja

mypy/meet.py

@@ -541,13 +541,24 @@ def _type_object_overlap(left: Type, right: Type) -> bool:
         return False
 
     if isinstance(left, CallableType) and isinstance(right, CallableType):
+        # We run is_callable_compatible in both directions, similar to the logic
+        # in is_unsafe_overlapping_overload_signatures
+        # See comments in https://github.com/python/mypy/pull/5476
         return is_callable_compatible(
             left,
             right,
             is_compat=_is_overlapping_types,
             is_proper_subtype=False,
             ignore_pos_arg_names=not overlap_for_overloads,
             allow_partial_overlap=True,
+        ) or is_callable_compatible(
+            right,
+            left,
+            is_compat=_is_overlapping_types,
+            is_proper_subtype=False,
+            ignore_pos_arg_names=not overlap_for_overloads,
+            check_args_covariantly=True,
+            allow_partial_overlap=True,
         )
 
     call = None

mypy/test/testtypes.py

@@ -8,7 +8,7 @@
 from mypy.erasetype import erase_type, remove_instance_last_known_values
 from mypy.indirection import TypeIndirectionVisitor
 from mypy.join import join_types
-from mypy.meet import meet_types, narrow_declared_type
+from mypy.meet import is_overlapping_types, meet_types, narrow_declared_type
 from mypy.nodes import (
     ARG_NAMED,
     ARG_OPT,
@@ -645,6 +645,20 @@ def assert_simplified_union(self, original: list[Type], union: Type) -> None:
         assert_equal(make_simplified_union(original), union)
         assert_equal(make_simplified_union(list(reversed(original))), union)
 
+    def test_generic_callable_overlap_is_symmetric(self) -> None:
+        any_type = AnyType(TypeOfAny.from_omitted_generics)
+        outer_t = TypeVarType("T", "T", TypeVarId(1), [], self.fx.o, any_type)
+        outer_s = TypeVarType("S", "S", TypeVarId(2), [], self.fx.o, any_type)
+        generic_t = TypeVarType("T", "T", TypeVarId(-1), [], self.fx.o, any_type)
+
+        callable_type = CallableType([outer_t], [ARG_POS], [None], outer_s, self.fx.function)
+        generic_identity = CallableType(
+            [generic_t], [ARG_POS], [None], generic_t, self.fx.function, variables=[generic_t]
+        )
+
+        assert is_overlapping_types(callable_type, generic_identity)
+        assert is_overlapping_types(generic_identity, callable_type)
+
     # Helpers
 
     def tuple(self, *a: Type) -> TupleType:

test-data/unit/check-narrowing.test

@@ -3995,6 +3995,25 @@ def f2(func: Callable[..., T], arg: str) -> T:
 [builtins fixtures/primitives.pyi]
 
 
+[case testNarrowGenericCallableEquality]
+# flags: --strict-equality --warn-unreachable
+from typing import Callable, TypeVar
+
+S = TypeVar("S")
+T = TypeVar("T")
+
+def identity(x: T) -> T:
+    return x
+
+def msg(cmp_property: Callable[[T], S]) -> None:
+    if cmp_property == identity:
+        # TODO: the swapping of these reveal's is not ideal
+        reveal_type(cmp_property)  # N: Revealed type is "def [T] (x: T`-1) -> T`-1"
+        reveal_type(identity)  # N: Revealed type is "def (T`-1) -> S`-2"
+        return
+[builtins fixtures/primitives.pyi]
+
+
 [case testPropagatedParentNarrowingMeet]
 # flags: --strict-equality --warn-unreachable
 from __future__ import annotations