Support TypeAliasType in a class scope (#17038)

Fixes
#16614 (comment)

Author: hamdanal

mypy/semanal.py

@@ -3657,7 +3657,7 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
             return False
 
         non_global_scope = self.type or self.is_func_scope()
-        if not pep_613 and isinstance(rvalue, RefExpr) and non_global_scope:
+        if not pep_613 and not pep_695 and isinstance(rvalue, RefExpr) and non_global_scope:
             # Fourth rule (special case): Non-subscripted right hand side creates a variable
             # at class and function scopes. For example:
             #

test-data/unit/check-type-aliases.test

@@ -1074,6 +1074,11 @@ TestType = TypeAliasType("TestType", Union[int, str])
 x: TestType = 42
 y: TestType = 'a'
 z: TestType = object()  # E: Incompatible types in assignment (expression has type "object", variable has type "Union[int, str]")
+
+class A:
+    ClassAlias = TypeAliasType("ClassAlias", int)
+xc: A.ClassAlias = 1
+yc: A.ClassAlias = ""  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
 [builtins fixtures/tuple.pyi]
 
 [case testTypeAliasTypeInvalid]