1 
2 template<typename _Callable>
3 struct Base
4 {
BaseBase5   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) {};
20