https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83445
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- Invoking Target(Source const&) is a user-defined conversion too. The conversion sequence requires binding a reference to the Source object and then converting it to a Target. In C++17 there is no call to Target(Target const&) or Target(Target&&) because the temporary returned by Source::operator Target() is materialized directly into an object of type Target. Clang agrees with GCC that Source::operator Target() is a better conversion, but I can't find the wording specifying that.
