https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106320
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> --- The problem is that join_view still requires its _InnerView to be default-constructible, but that's a transform_view holding a non-default-constructible callable. For gcc-11 this isn't a problem because of r11-8477-ge1489a3d613b6f implementing P2328, which makes join_view uses non-propagating-cache for _M_inner. That isn't on the gcc-10 branch. I'm tempted to revert the PR 103904 backport on the branch. I was never very happy about backporting it.
