https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81211
--- Comment #3 from Stephen Kell <srk31 at srcf dot ucam.org> --- I agree that this message is better (my bad for not trying a supported release). It'd be nice if I didn't have to guess that the reason it's not callable is that I omitted the copy constructor. The rules for when constructors are defaulted/deleted are baroque enough to catch anybody out from time to time.
