https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78830
--- Comment #2 from Andrzej Krzemienski <akrzemi1 at gmail dot com> --- Sorry, you are correct. As per [res.on.functions], passing types that do not satisfy the requirements is a UB, and therefore a well formed program. GCC is standard-compliant in this respect. So, I have lost my strongest argument, but I still feel this check should not be "opt-in". As a QOI matter, could a standard-allowed UB be implemented as a compile-time failure?
