1  
       2  template<typename _Callable>
       3  struct Base
       4  {
       5    Base (const _Callable &)
       6      requires true
       7    {}
       8  };
       9  
      10  template<typename _Callable>
      11  struct Derived :  Base<_Callable>
      12  {
      13    using Base<_Callable>::Base;
      14  };
      15  
      16  template<typename _Callable>
      17  Derived (_Callable) -> Derived<_Callable>;
      18  
      19  inline Derived all = [] (auto&& __r) {};