Fix sentinel identity loss through generic substitution and inference

mypy/erasetype.py

@@ -246,6 +246,10 @@ class LastKnownValueEraser(TypeTranslator):
     def visit_instance(self, t: Instance) -> Type:
         if not t.last_known_value and not t.args:
             return t
+        if t.last_known_value is not None and t.last_known_value.is_sentinel_literal():
+            # Sentinel values (PEP 661) have no other way to identify themselves than
+            # via their literal, unlike e.g. enum members, so it must be preserved.
+            return t
         return t.copy_modified(args=[a.accept(self) for a in t.args], last_known_value=None)
 
     def visit_type_alias_type(self, t: TypeAliasType) -> Type:

mypy/expandtype.py

@@ -244,6 +244,10 @@ def visit_type_var(self, t: TypeVarType) -> Type:
             t = t.copy_modified(upper_bound=t.upper_bound.accept(self))
         repl = self.variables.get(t.id, t)
         if isinstance(repl, ProperType) and isinstance(repl, Instance):
+            if repl.last_known_value is not None and repl.last_known_value.is_sentinel_literal():
+                # Sentinel values (PEP 661) have no other way to identify themselves than
+                # via their literal, unlike e.g. enum members, so it must survive expansion.
+                return repl
             # TODO: do we really need to do this?
             # If I try to remove this special-casing ~40 tests fail on reveal_type().
             return repl.copy_modified(last_known_value=None)

test-data/unit/check-sentinels.test

@@ -181,11 +181,28 @@ from typing_extensions import sentinel, assert_type
 MISSING = sentinel("MISSING")
 ALIAS = MISSING
 
-assert_type(ALIAS, sentinel)
+# The value still identifies as the same sentinel...
+assert_type(ALIAS, MISSING)
 
 def func(x: int | MISSING = MISSING) -> None:
     pass
 
 func(MISSING)
-func(ALIAS)  # E: Argument 1 to "func" has incompatible type "Sentinel"; expected "int | MISSING"
+func(ALIAS)
+
+# ...but the reassignment does not make ALIAS usable as a type alias.
+def uses_alias_as_type(x: ALIAS) -> None:  # E: Variable "__main__.ALIAS" is not valid as a type \
+        # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
+    pass
 [builtins fixtures/tuple.pyi]
+
+[case testSentinelPreservedThroughGenericSubstitution]
+from typing import assert_type
+from typing_extensions import sentinel
+
+Unknown = sentinel("Unknown")
+
+def func(d: dict[str, str]) -> None:
+    var = d.get("key", Unknown)
+    assert_type(var, str | Unknown)
+[builtins fixtures/dict-full.pyi]