Unsoundly narrow away from None with custom eq

In #20754 we see some primer hits where this unsoundness is beneficial

Author: hauntsaninja

mypy/checker.py

@@ -6746,7 +6746,12 @@ def narrow_type_by_identity_equality(
                         if i == j:
                             continue
                         # If we compare to a target with custom __eq__, we cannot narrow at all
-                        or_if_maps.append({})
+                        if is_overlapping_none(expr_type) and not is_overlapping_none(
+                            operand_types[j]
+                        ):
+                            or_if_maps.append({operands[i]: remove_optional(expr_type)})
+                        else:
+                            or_if_maps.append({operands[i]: expr_type})
                         continue
                     target_type = operand_types[j]
                     if should_coerce_literals:

mypy/types_utils.py

@@ -26,6 +26,7 @@
     TypeAliasType,
     TypeType,
     TypeVarType,
+    UninhabitedType,
     UnionType,
     UnpackType,
     flatten_nested_unions,
@@ -134,6 +135,8 @@ def remove_optional(typ: Type) -> Type:
         return UnionType.make_union(
             [t for t in typ.items if not isinstance(get_proper_type(t), NoneType)]
         )
+    elif isinstance(typ, NoneType):
+        return UninhabitedType()
     else:
         return typ
 

test-data/unit/check-narrowing.test

@@ -1008,6 +1008,22 @@ def f(x: Custom | None, y: int | None):
 [case testNarrowingCustomEqualityUnion4]
 # flags: --strict-equality --warn-unreachable
 from __future__ import annotations
+
+class Custom:
+    def __eq__(self, other: object) -> bool:
+        return True
+
+def f(x: Custom | None, y: Custom):
+    if x == y:
+        # We unsoundly special case None and narrow x to Custom here
+        reveal_type(x)  # N: Revealed type is "__main__.Custom"
+    else:
+        reveal_type(x)  # N: Revealed type is "__main__.Custom | None"
+[builtins fixtures/primitives.pyi]
+
+[case testNarrowingCustomEqualityUnion5]
+# flags: --strict-equality --warn-unreachable
+from __future__ import annotations
 from typing import Any
 
 class Custom1: