Reject sentinels as types when defined within a class nested in a function

Signed-off-by: Edgar Ramírez Mondragón <edgarrm358@gmail.com>

Author: edgarrmondragon

mypy/semanal.py

@@ -3402,7 +3402,7 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
 
     def is_sentinel_declaration(self, s: AssignmentStmt) -> bool:
         """Does this assignment define a PEP 661 sentinel singleton?"""
-        if self.is_func_scope() or s.unanalyzed_type is not None:
+        if self.is_nested_within_func_scope() or s.unanalyzed_type is not None:
             return False
         if len(s.lvalues) != 1 or not isinstance(s.lvalues[0], NameExpr):
             return False

test-data/unit/check-sentinels.test

@@ -65,6 +65,17 @@ def outer() -> None:
                                       # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
 [builtins fixtures/tuple.pyi]
 
+[case testSentinelClassNestedInFunctionNotTypeExpression]
+from typing_extensions import Sentinel
+
+def outer() -> None:
+    class C:
+        MISSING = Sentinel("MISSING")
+
+    def inner(x: C.MISSING) -> None: ...  # E: Variable "__main__.C@4.MISSING" is not valid as a type \
+                                          # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
+[builtins fixtures/tuple.pyi]
+
 [case testSentinelImplicitlyFinal]
 from typing_extensions import Sentinel