Don't use equality to narrow when value is IntEnum/StrEnum (#17866)

IntEnum/StrEnum values compare equal to the corresponding int/str
values, which breaks the logic we use for narrowing based on equality to
a literal value. Special case IntEnum/StrEnum to avoid the incorrect
behavior.

Fix #17860.

Author: JukkaL

mypy/typeops.py

@@ -1022,6 +1022,14 @@ def custom_special_method(typ: Type, name: str, check_all: bool = False) -> bool
     """
     typ = get_proper_type(typ)
     if isinstance(typ, Instance):
+        if (
+            typ.type.is_enum
+            and name in ("__eq__", "__ne__")
+            and any(base.fullname in ("enum.IntEnum", "enum.StrEnum") for base in typ.type.mro)
+        ):
+            # IntEnum and StrEnum values have non-straightfoward equality, so treat them
+            # as if they had custom __eq__ and __ne__
+            return True
         method = typ.type.get(name)
         if method and isinstance(method.node, (SYMBOL_FUNCBASE_TYPES, Decorator, Var)):
             if method.node.info:

test-data/unit/check-narrowing.test

@@ -2130,3 +2130,79 @@ else:
 
 [typing fixtures/typing-medium.pyi]
 [builtins fixtures/ops.pyi]
+
+[case testNarrowingWithIntEnum]
+# mypy: strict-equality
+from __future__ import annotations
+from typing import Any
+from enum import IntEnum, StrEnum
+
+class IE(IntEnum):
+    X = 1
+    Y = 2
+
+def f1(x: int) -> None:
+    if x == IE.X:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+    else:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+    if x != IE.X:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+    else:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+
+def f2(x: IE) -> None:
+    if x == 1:
+        reveal_type(x)  # N: Revealed type is "__main__.IE"
+    else:
+        reveal_type(x)  # N: Revealed type is "__main__.IE"
+
+def f3(x: object) -> None:
+    if x == IE.X:
+        reveal_type(x)  # N: Revealed type is "builtins.object"
+    else:
+        reveal_type(x)  # N: Revealed type is "builtins.object"
+
+def f4(x: int | Any) -> None:
+    if x == IE.X:
+        reveal_type(x)  # N: Revealed type is "Union[builtins.int, Any]"
+    else:
+        reveal_type(x)  # N: Revealed type is "Union[builtins.int, Any]"
+
+def f5(x: int) -> None:
+    if x is IE.X:
+        reveal_type(x)  # N: Revealed type is "Literal[__main__.IE.X]"
+    else:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+    if x is not IE.X:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+    else:
+        reveal_type(x)  # N: Revealed type is "Literal[__main__.IE.X]"
+[builtins fixtures/primitives.pyi]
+
+[case testNarrowingWithStrEnum]
+# mypy: strict-equality
+from enum import StrEnum
+
+class SE(StrEnum):
+    A = 'a'
+    B = 'b'
+
+def f1(x: str) -> None:
+    if x == SE.A:
+        reveal_type(x)  # N: Revealed type is "builtins.str"
+    else:
+        reveal_type(x)  # N: Revealed type is "builtins.str"
+
+def f2(x: SE) -> None:
+    if x == 'a':
+        reveal_type(x)  # N: Revealed type is "__main__.SE"
+    else:
+        reveal_type(x)  # N: Revealed type is "__main__.SE"
+
+def f3(x: object) -> None:
+    if x == SE.A:
+        reveal_type(x)  # N: Revealed type is "builtins.object"
+    else:
+        reveal_type(x)  # N: Revealed type is "builtins.object"
+[builtins fixtures/primitives.pyi]