Improve sentinel narrowing

mypy/typeops.py

@@ -37,6 +37,7 @@
 from mypy.types import (
     ELLIPSIS_TYPE_NAMES,
     NOT_IMPLEMENTED_TYPE_NAMES,
+    SENTINEL_TYPE_NAMES,
     AnyType,
     CallableType,
     ExtraAttrs,
@@ -1056,6 +1057,7 @@ def is_singleton_identity_type(typ: ProperType) -> bool:
             (typ.type.is_enum and len(typ.type.enum_members) == 1)
             or (typ.type.fullname in ELLIPSIS_TYPE_NAMES)
             or (typ.type.fullname in NOT_IMPLEMENTED_TYPE_NAMES)
+            or (typ.type.fullname in SENTINEL_TYPE_NAMES)
         )
     if isinstance(typ, LiteralType):
         return typ.is_enum_literal() or typ.is_sentinel_literal() or isinstance(typ.value, bool)

test-data/unit/check-sentinels.test

@@ -36,12 +36,22 @@ MISSING = Sentinel("MISSING")
 SPECIAL = Sentinel("SPECIAL")
 
 def func(x: int | MISSING | SPECIAL) -> None:
-    # We could reasonably do narrowing here, but currently we're pretty conservative
-    # about narrowing on ==.
     if x == MISSING:
-        assert_type(x, int | MISSING | SPECIAL)
+        assert_type(x, MISSING)
+    else:
+        assert_type(x, int | SPECIAL)
+[builtins fixtures/ops.pyi]
+
+[case testSentinelNarrowingInSequence]
+from typing_extensions import assert_type, Sentinel
+
+MISSING = Sentinel("MISSING")
+
+def func(var: str | MISSING | None) -> None:
+    if var in (MISSING, None):
+        assert_type(var, MISSING | None)
     else:
-        assert_type(x, int | MISSING | SPECIAL)
+        assert_type(var, str)
 [builtins fixtures/tuple.pyi]
 
 [case testSentinelSameReprDistinctTypes]

test-data/unit/lib-stub/typing_extensions.pyi

@@ -44,7 +44,6 @@ Never: _SpecialForm
 
 class Sentinel:
     def __init__(self, name: str, repr: str | None = None) -> None: ...
-    def __eq__(self, other: object) -> bool: ...
 sentinel = Sentinel
 
 TypeVarTuple: _SpecialForm