I created a trait struct FnOnArg to see if Fn can call on Arg

template<typename Fn, typename Arg, typename=void>
struct FnOnArg: std::false_type {};

template<typename Fn, typename Arg>
struct FnOnArg<Fn, Arg, std::void_t<decltype(std::declval<Fn>()(std::declval<Arg>()))>>: std::true_type {};

It can only work on Fn with one argument
I want to create FnOnArgs like

template<typename Fn, typename... Args, typename=void>
struct FnOnArgs;

but variadic and default template argument both should be the last argument
Any workaround to achieve FnOnArgs?