https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40177
--- Comment #2 from Andrew Pinski ---
GCC accept the original testcase now in C++20 mode since GCC 9+ and reject it
in C++98, C++11, C++14, and C++17 modes.
Clang also rejects it in < C++20 but with a decent error message:
:2:14: error: use of fu
--- Comment #1 from jason at gcc dot gnu dot org 2009-11-05 22:11 ---
I agree: before we can do ADL we need to know that f is a template name, so we
need another declaration of f to be found by unqualified lookup, i.e. something
like
template void f(T,T);
It is correct to do ADL once
