Better solution for is_subtype check

Author: cdce8p

mypy/subtypes.py

@@ -290,9 +290,6 @@ def _is_subtype(
         # ErasedType as we do for non-proper subtyping.
         return True
 
-    if not proper_subtype and isinstance(right, TypeVarType) and right.has_default():
-        return left.accept(SubtypeVisitor(right.default, subtype_context, proper_subtype))
-
     if isinstance(right, UnionType) and not isinstance(left, UnionType):
         # Normally, when 'left' is not itself a union, the only way
         # 'left' can be a subtype of the union 'right' is if it is a
@@ -621,8 +618,9 @@ def check_mixed(
             if call:
                 return self._is_subtype(call, right)
             return False
-        else:
-            return False
+        if isinstance(right, TypeVarType) and right.has_default():
+            return self._is_subtype(left, right.default)
+        return False
 
     def visit_type_var(self, left: TypeVarType) -> bool:
         right = self.right