Forwarding of args in xchar.h

[Boddlnagg]

In the same vein as #2462, I think that format_to_n in xchar.h should take args as Args&&... instead of const Args&.... For format_to in xchar.h this is already the case.

make_wformat_args should probably also be changed to match make_format_args in forwarding the args.

[Epixu]

I'm currently working on a similar issue around make_format_args
They are currently taken as mutable Args&...
The comment above states that "Arguments are taken by lvalue references to avoid some lifetime issues."
And this is an implementation detail, so I suppose it's our responsibility to adapt.
Although, make_format_args is used extensively in custom formatters afaik, so the devs might still reconsider it having the proper interface eventually.

Or I might be completely missing something. Here's a custom formatter I wrote for a reference:

template<Deep T>
struct formatter<T> {
   template<class CONTEXT>
   constexpr auto parse(CONTEXT& ctx) {
      return ctx.begin();
   }

   template<class CONTEXT>
   auto format(T const& element, CONTEXT& ctx) {
      const auto asText = static_cast<Debug>(element);
      return fmt::vformat_to(ctx.out(), "{}", fmt::make_format_args(static_cast<std::string_view>(asText)));
   }
};

This currently causes error C2664 on MSVC

PP: I think this error was introduced in v. 10

[vitaut]

@Epixu, make_format_args should only be used in custom formatting functions, not in custom formatters. You should use format_to instead of vformat_to:

      return fmt::format_to(ctx.out(), "{}", static_cast<std::string_view>(asText));

[vitaut]

Fixed format_to signature in a425e0f.