https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24847
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> --- Overload resolution considers both constructors, to decide which one v(i) should use. Performing overload resolution on foo<int, 5, b>::foo(foo<int, 0, b>) will cause the instantiation of struct foo<int, 0, b> to see if there is a valid conversion sequence from int to foo<int, 0, b>. Instantiating that type causes the instantiation of foo<int, 0, b>::foo(foo<int, 0, b>) which is indeed an invalid copy constructor. So the problem is not that it thinks the second constructor is a copy constructor of foo<int, 5, b>, obviously it's not. But it is an invalid copy constructor for foo<int, 0, b> which gets implicitly instantiated. I don't know how Clang accepts this.
