1  
       2  template<typename _Iterator, typename>
       3  struct Trait;
       4  
       5  template<typename _Iterator>
       6  struct Trait<_Iterator, void> {};
       7  
       8  template<typename _Iterator>
       9  requires  true
      10  struct Trait<_Iterator, void>
      11  {
      12    template<typename _Iter> struct __diff {};
      13    
      14    template<typename _Iter> requires true struct __diff<_Iter> {};
      15  };