Fix custom equality handling for membership narrowing in static containers

This was an issue in the new feature #21461

In narrow_type_by_identity_equality we check for custom equality before
coercing to literals, and this was done the wrong way round here

Fixes #21703

Author: hauntsaninja

mypy/checker.py

@@ -6858,9 +6858,13 @@ def comparison_type_narrowing_helper(self, node: ComparisonExpr) -> tuple[TypeMa
                         for known_item in container_item_types:
                             # Match the should_coerce_literals logic from narrow_type_by_identity_equality
                             p_known_item = get_proper_type(known_item)
-                            if is_literal_type_like(p_known_item) or (
-                                isinstance(p_known_item, Instance) and p_known_item.type.is_enum
-                            ):
+                            if (
+                                is_literal_type_like(p_known_item)
+                                or (
+                                    isinstance(p_known_item, Instance)
+                                    and p_known_item.type.is_enum
+                                )
+                            ) and not has_custom_eq_checks(p_known_item):
                                 known_item = coerce_to_literal(known_item)
                             if_map, else_map = self.narrow_type_by_identity_equality(
                                 "==",

test-data/unit/check-narrowing.test

@@ -3335,6 +3335,24 @@ def narrow_dict(x: Literal['a', 'b', 'c'], t: dict[Literal['a', 'b'], int]):
 [builtins fixtures/primitives.pyi]
 
 
+[case testNarrowCustomEqEnumInLiteralContainer]
+# flags: --strict-equality --warn-unreachable
+# https://github.com/python/mypy/issues/21703
+from enum import Enum
+
+class E(Enum):
+    foo = 1
+    bar = 2
+
+    def __eq__(self, other: object) -> bool:
+        return True
+
+def f(x: int) -> None:
+    if x in [E.foo, E.bar]:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+[builtins fixtures/list.pyi]
+
+
 [case testNarrowingLiteralInLiteralContainer]
 # flags: --strict-equality --warn-unreachable
 from typing import Literal