https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113074
--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> --- It looks like libc++ did it for this reason: [libc++] Fix a hard error in `contiguous_iterator<NoOperatorArrowIter>`. Evaluating `contiguous_iterator` on an iterator that satisfies all the constraints except the `to_address` constraint and doesn't have `operator->` defined results in a hard error. This is because instantiating `to_address` ends up instantiating templates dependent on the given type which might lead to a hard error even in a SFINAE context. Differential Revision: https://reviews.llvm.org/D130835 As I said in comment 7, LWG considered this case and it was pointed out that the problem described can only occur if a type defines iterator_concept = contiguous_iterator; but then fails to actually provide the operations needed for a contiguous iterator (i.e. either a pointer_traits specialization with to_address or a sane operator->()). A SFINAE-friendly std::to_address as implemented in libc++ means that such an iterator will fail to satisfy std::contiguous_iterator and will silently degrade to satosfying std::random_access_iterator only. It's not at all clear to me that silently degrading such an iterator (which very explicitly claims to be a contiguous iterator by defining iterator_concept to say so) would be an improvement. I'd rather get an error telling me the thing I thought was a contiguous iterator was not actually.
