Add static_assert for better error messages

[David Peter]

Obviously, we can not create a painless::Parameter<T> for any T. We should think about the proper constraint and add a static_assert such that users will see a better error message.

For now, we probably require existence of ostream& operator<< and istream& operator>> for "serialization" and "deserialization".

[Henri Menke]

Just reuse the is_detected bits that I wrote for dbg.
https://github.com/sharkdp/dbg-macro/blob/4c59a8662161f4b3d78361cc1e677968a80fc170/dbg.h#L252-L286

The ostream detection is already there.
https://github.com/sharkdp/dbg-macro/blob/4c59a8662161f4b3d78361cc1e677968a80fc170/dbg.h#L327-L332

And istream detection is just straight-forward.

template <typename T>
using istream_operator_t =
    decltype(std::declval<std::istream&>() >> std::declval<T>());

template <typename T>
struct has_istream_operator : is_detected<istream_operator_t, T> {};

Then you can just do

static_assert(detail::has_ostream_operator<const T&>::value,
              "Type does not support the << ostream operator");

but you probably want to use the tag dispatch that I used in dbg
https://github.com/sharkdp/dbg-macro/blob/4c59a8662161f4b3d78361cc1e677968a80fc170/dbg.h#L345-L366

[David Peter]

Just reuse the is_detected bits that I wrote for dbg.
https://github.com/sharkdp/dbg-macro/blob/4c59a8662161f4b3d78361cc1e677968a80fc170/dbg.h#L252-L286

Exactly what I was thinking about when creating this ticket 👍