Add __class__ assignment check to TypeChecker.visit_assignment_stmt

Implemented the check inside TypeChecker.visit_assignment_stmt to disallow assignments to '__class__', as such assignments are unsafe.

The check correctly uses self.fail() following mypy's error reporting conventions, and is located after initial assignment processing to align with the design of TypeChecker.

Previous redundant logic inside VarAssignVisitor.visit_assignment_stmt has been removed.

This change addresses issue #7724.

Author: lsrafael13

mypy/checker.py

@@ -3077,7 +3077,12 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
         if not (s.is_alias_def and self.is_stub):
             with self.enter_final_context(s.is_final_def):
                 self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
-
+        
+        for lv in s.lvalues:
+            if isinstance(lv, MemberExpr):
+                if lv.name == '__class__':
+                    self.fail("Assignment to '__class__' is unsafe and not allowed", lv)
+        
         if s.is_alias_def:
             self.check_type_alias_rvalue(s)
 
@@ -8880,9 +8885,6 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
         self.lvalue = True
         for lv in s.lvalues:
             lv.accept(self)
-            if isinstance(lv, MemberExpr):
-                if lv.name == '__class__':
-                    self.fail("Assignment to '__class__' is unsafe and not allowed", lv)
         self.lvalue = False
 
     def visit_name_expr(self, e: NameExpr) -> None: