Fix bug when narrowing union containing custom eq against custom eq (#20754)

I rewrote the logic for custom equality in
#20643 . This is a soundness bug in
that rewrite.

Fixes #20750

Author: hauntsaninja

mypy/checker.py

@@ -6736,13 +6736,17 @@ def narrow_type_by_identity_equality(
             or_else_maps: list[TypeMap] = []
             for expr_type in union_expr_type.items:
                 if has_custom_eq_checks(expr_type):
-                    # Always include union items with custom __eq__ in the type
+                    # Always include the union items with custom __eq__ in the type
                     # we narrow to in the if_map
                     or_if_maps.append({operands[i]: expr_type})
 
                 expr_type = coerce_to_literal(try_expanding_sum_type_to_union(expr_type, None))
                 for j in expr_indices:
                     if j in custom_eq_indices:
+                        if i == j:
+                            continue
+                        # If we compare to a target with custom __eq__, we cannot narrow at all
+                        or_if_maps.append({})
                         continue
                     target_type = operand_types[j]
                     if should_coerce_literals:

test-data/unit/check-narrowing.test

@@ -1005,6 +1005,50 @@ def f(x: Custom | None, y: int | None):
         reveal_type(y)  # N: Revealed type is "builtins.int | None"
 [builtins fixtures/primitives.pyi]
 
+[case testNarrowingCustomEqualityUnion4]
+# flags: --strict-equality --warn-unreachable
+from __future__ import annotations
+from typing import Any
+
+class Custom1:
+    def __eq__(self, other: object) -> bool:
+        raise
+
+class Custom2:
+    def __eq__(self, other: object) -> bool:
+        raise
+
+def f(x: Custom1 | int, y: Custom2 | int):
+    if x == y:
+        reveal_type(x)  # N: Revealed type is "__main__.Custom1 | builtins.int"
+        reveal_type(y)  # N: Revealed type is "__main__.Custom2 | builtins.int"
+    else:
+        reveal_type(x)  # N: Revealed type is "__main__.Custom1 | builtins.int"
+        reveal_type(y)  # N: Revealed type is "__main__.Custom2 | builtins.int"
+[builtins fixtures/primitives.pyi]
+
+[case testNarrowingCustomEqualitySubclass]
+# flags: --strict-equality --warn-unreachable
+from __future__ import annotations
+from typing import Any
+
+class Custom:
+    def __eq__(self, other: object) -> bool:
+        raise
+
+class CustomSub(Custom):
+    def __eq__(self, other: object) -> bool:
+        raise
+
+def f(x: Custom, y: CustomSub):
+    if x == y:
+        reveal_type(x)  # N: Revealed type is "__main__.Custom"
+        reveal_type(y)  # N: Revealed type is "__main__.CustomSub"
+    else:
+        reveal_type(x)  # N: Revealed type is "__main__.Custom"
+        reveal_type(y)  # N: Revealed type is "__main__.CustomSub"
+[builtins fixtures/tuple.pyi]
+
 [case testNarrowingUnreachableCases]
 # flags: --strict-equality --warn-unreachable
 from typing import Literal, Union