Fix assign to type

Author: cdce8p

mypy/subtypes.py

@@ -628,6 +628,8 @@ def visit_type_var(self, left: TypeVarType) -> bool:
             return True
         if left.values and self._is_subtype(UnionType.make_union(left.values), right):
             return True
+        if left.has_default():
+            return self._is_subtype(left.default, self.right)
         return self._is_subtype(left.upper_bound, self.right)
 
     def visit_param_spec(self, left: ParamSpecType) -> bool: