https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- The working draft says: text_encoding::aliases_view models copyable, ranges::view, ranges::random_access_range, and ranges::borrowed_range. It specifically says copyable and not semiregular, which suggests to me that it isn't required to model semiregular, so isn't required to be default-initializable. I think libstdc++ is correct, and the working draft should be clarified.
