https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86521
--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> --- The cast is ambiguous To construct a 'base', we consider the two constructors 1) base(const base&); 2) base(base&&); for each of them we could convert the argument by either 3) operator U () && 4) operator U const& () const& For #1 we want to convert to const base&. For direct reference binding, #4 is the only candidate, and it is viable. For #2 we want to convert to base&&, only #3 is a candidate for direct reference binding, and it is viable. The two user-defined conversion sequences are not comparable because they use different conversion operators, so the initialization is ambiguous.
