Hi!
 
>
> As one of the "I am just a regular user, not a compiler guru" lurkers on this list, I'd just like to mention that anything that involves call-site annotation is a  great source of confusion for regular users, especially when the answer to "Why?" is "obscure reason X" as it would be in this case. Shifting the responsibility to the caller is just as ineffective as documenting the issue in a manual.


Let me explain why it might be a good idea to have a call-site annotation for ADL. The reason is not obscure, the annotation designates an unusual function call - a customization point. Moreover, until recently it was a recommended practice to annotate such calls in the following way:
using std::swap;
swap(a, b);

it is effectively an annotation, but a strange one, leaving it without comment would raise a question from a regular user. Now we can write
std::ranges::swap(a, b); of course, however, design of customization points of std::ranges currently relies on intricacies of ADL name lookup.
So, I would argue that the annotation would be useful in the implementation of such wrappers as std::ranges::swap, std::ranges::begin etc.

Annotation is not bad, if it actually helps a user to understand that something unusual is going on, then after consulting language reference or just google a user can understand what exactly is going on.  

Second point is that it would be rare, most of the functions are not customization points.

Third point is that such annotation allows what is currently not possible. If you look up the definition of range-based for loop in the standard you'd see that it is impossible to write its implementation in plain C++. To find begin and end for a container it performs only ADL w/o usual name lookup. An annotated call would do exactly that.

Regards,
Gregory

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