[R&D] Check compile time improvement with alternative to std::declval

[SadiinsoSnowfall]

Eric Niebler on twitter seems to have found an alternative to std::declval that compiles at least twice as fast under clang 17.

template ‹class T>
using identity_t = T;

template ‹class, class = void>
extern identity_t<void (*)() noexcept> declval;

template <class T>
extern identity_t<T && (*)() noexcept> declval<T, void_t<T &&>>;

As well as another implementation which is said to compile up to three time faster but cannot handle some function types:

template <class T>
using identity_t = T;

template <class T>
extern identity_t<T&& (*)() noexcept> declval;

template <>
constexpr identity_t<void (*)() noexcept> declval<void>;

template <>
constexpr identity_t<void (*)() noexcept> declval<void const>;

template <>
constexpr identity_t<void (*)() noexcept> declval<void volatile>;

template <>
constexpr identity_t<void (*)() noexcept> declval<void const volatile>;

This might be something we want to look into.