How to use using-declarations in constraint

Question

Is there an alternative to have some using -declarations in concept/constraint? Something like:

template <typename T>
concept has_begin_v0 = requires (T t)
{
    using std::begin; // KO
    begin(t);
    /*..*/
};

The possible ways I have found are:

• use intermediate namespace

   namespace detail { using std::begin; template <typename T> concept has_begin_v1 = requires (T t) { begin(t); }; } using detail::has_begin_v1;

  Introduces extra namespace:-(

• using SFINAEd lambda:

   template <typename T> concept has_begin_v2 = requires (T t) { [](){ using std::begin; return [](auto&& inner) -> std::void_t<decltype(begin(inner))> {}; }()(t); };

  Not a fine syntax.

• use "disjunction" of qualified call and adl call:

   template <typename T> concept has_std_begin = requires (T t) { std::begin(t); }; template <typename T> concept has_adl_begin = requires (T t) { begin(t); }; template <typename T> concept has_begin_v3 = has_std_begin<T> || has_adl_begin<T>;

  The nicer IMO, but seems not to scale nor allow to go further (is begin(t) comparable with end(t) ?).

Demo

Answer 1

You should generally avoid having to do the using std::begin trick. Since you're already operating in C++20, you can use the ranges customization point object std::ranges::begin . It does the ADL gymnastics you need on a direct call, so you don't need using std::begin .