I think there's value in that mode, for library writers (not libc++) who really care about finding every unintentional ADL in their whole library. The standard library is designed not-to-work with types like [Holder<Incomplete>](https://quuxplusone.github.io/blog/2019/09/26/uglification-doesnt-stop-adl/), but someone else's library might be designed to work even in that case, and then they'd want to hear about ADL lookups for things like operator,. Besides, it's just 1 extra line of code in the patch, isn't it?

However, I now think I may not understand how this check works. I thought it looked for unqualified calls (even in templates) that "may" use ADL, but now that I look again at the tests, it seems to trigger only on concrete calls (in concrete template instantiations) that "do" use ADL, which sounds still useful but much less comprehensive than I had thought.

I think it would catch

template<class T> void foo(T t) { t, 0; }
struct S { friend void operator,(S, int); };
template void foo(S);

but not

template<class T> void foo(T t) { t, 0; }
struct S { friend void operator,(S, int); };
template void foo(int);

or

template<class T> void foo(T t) { t, 0; }

is that right?