A.O.D. has written: -- 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. --
Why not just going further: -fno-adl
That would certainly "rock the boat". In my not-a-compiler-writer naivete, I might think, contrary to A.O.D. (gasp!) a lot of code will work unbroken with that switch.? Especially that silent not-minority not using std lib.
ADL is used whenever you write
std::string s;
auto t = s + s; // ADL on operator+
std::cout << s; // ADL on operator<<
swap(s, s); // ADL on swap
In particular, the "std::swap two-step" is used inside all swap-based STL algorithms, such as std::rotate and std::sort.
So you can't actually turn it off, because then you'd break the world (because approximately every C++ program depends on ADL somewhere). But you could issue a warning about it outside of system headers, just to get a sense of the magnitude of the issue.
–Arthur