support c++17 and c++20

[MatchX]

https://github.com/MatchX/cpp_threadpool

[AmanjitPREA]

As @MatchX already mentioned...

#include <type_traits>
...
decltype( auto ) enqueue( F&& f, Args&& ... args );
...
decltype( auto ) ThreadPool::enqueue( F&& f, Args&& ... args ) { using return_type = std::invoke_result_t<std::decay_t<F>, std::decay_t<Args>...>;

seems to do the trick