[Someone wrote:]
> 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);
I agree with you both. :) One of the big problems with ADL — and unfortunately the biggest problem with ADL is that we can't get rid of it ;) — is that it's a tool that can be used in several different ways, for several different purposes. Sometimes the programmer wants to say "This function is intended for use with ADL"; sometimes the programmer wants to say "This call-site intends to use ADL." Either intent can be valid, at different places in the same program.
See my blog post "What is the std::swap two-step?"
where I compare plain old ADL to a pure virtual function, and ADL-with-the-two-step to a non-pure, yet virtual, function.
I'd be interested to see a compiler patch to warn whenever "ADL lookup occurred, on a non-operator, and ADL found a best-matching candidate which would not have been found by regular unqualified lookup." It couldn't be used in production because approximately every C++ program depends on ADL somewhere. But it could give us a sense of the magnitude of the issue.
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.
IIRC, std::ranges::begin(x) and the other Ranges CPOs are also specified to perform only ADL without the usual unqualified name lookup. They have a way to approximate that, by using a detail namespace and a "poison pill"; but it's not 100% perfect and of course it's more expensive in terms of compile time than a simple built-in annotation would be.
my $.02,
–Arthur