https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101113
TC <rs2740 at gmail dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |rs2740 at gmail dot com --- Comment #2 from TC <rs2740 at gmail dot com> --- https://eel.is/c++draft/class.copy.ctor#5 I don't think this code is valid. The constraint (which isn't checked until overload resolution time anyway) can't suppress the outright ill-formedness of the declaration.