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.