My favorite is "template<auto>". An universal smart pointer:

    template <typename T,auto f> struct smart
    {
     T* pointer=nullptr;

     ~smart(){f(pointer);}
    };

    smart<char,free> buffer={malloc(...)};
    smart<FILE*,fclose> file={fopen(...)};
    ...

Something to keep in the "potential subset".

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p012...