Fix #20821: Check __qualname__ assignment type in class body

mypy/semanal.py

@@ -3325,7 +3325,11 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
             # This should be safe as generally semantic analyzer is idempotent.
             with self.allow_unbound_tvars_set():
                 s.rvalue.accept(self)
-
+        if self.is_class_scope() and not self.is_stub_file and not self.is_typeshed_stub_file:
+            for lvalue in s.lvalues:
+                if isinstance(lvalue, NameExpr) and lvalue.name == "__qualname__":
+                    if not isinstance(s.rvalue, StrExpr):
+                        self.fail('"__qualname__" must be str', s)
         # The r.h.s. is now ready to be classified, first check if it is a special form:
         special_form = False
         # * type alias

test-data/unit/check-classes.test

@@ -9412,3 +9412,9 @@ from typ import NT
 def f() -> NT:
     return NT(x='')
 [builtins fixtures/tuple.pyi]
+
+[case test_qualname_must_be_str]
+class X:
+    __qualname__ = 5
+[out]
+main:2: error: "__qualname__" must be str