On Mon, Jun 8, 2020 at 1:35 PM Ville Voutilainen via SG10 <sg10@lists.isocpp.org> wrote:
On Mon, 8 Jun 2020 at 23:30, Gabriel Dos Reis via SG10
<sg10@lists.isocpp.org> wrote:
>
> At the time, I think I would have thought one was not needed, but that could be wrong.
>
>
>
> Indeed, this sounds to be like an overkill application of feature test macros, but…

Well, this macro gives users with the use case I depicted some more choices:
1) use a work-around that is just.. ..ghastly
2) provide an ADL-only customization point if and only if it can be
written without ghastliness
3) just provide a customization point that is not ADL-only
4) do (1) or (3) if (2) is not available

I thought the usual way you write ADL-only calls is

template<typename T> void f(T t) {
  using somewhere::name;
  name(t); // ADL-only call with fallback to somewhere::name
}

(eg, this is how I think we would recommend that people call 'begin' and 'end': "using std::begin; begin(x);"). I think that works equally well if 'somewhere' contains a function template, even if you don't have anything to put in namespace 'somewhere':

namespace somewhere {
  template<typename T> void name(...) = delete; // specialization missing
}
template<typename T> void f(T t) {
  using somewhere::name;
  name<int>(t); // ADL-only call with fallback to somewhere::name
}

This works without a feature test macro, and is more reliable than relying on the new feature. (For example, the above pattern is not broken if someone incautiously adds a non-function, non-function-template 'name' to a scope enclosing 'f', whereas use of the new feature would be broken by that.)