https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91212
--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> --- I think this is a bug in pre-P1825R0 handling of the restriction that the first overload resolution fails "if the type of the first parameter of the selected constructor is not an rvalue reference to the object's type (possibly cv-qualified)". But that restriction was removed by P1825R0, and I think this demonstrates a problem with that. Testing a patch.
