https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93978
--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:6d082cd90131a9c0ce3142217e84194a5bf0de27 commit r10-7066-g6d082cd90131a9c0ce3142217e84194a5bf0de27 Author: Patrick Palka <ppa...@redhat.com> Date: Thu Mar 5 10:04:06 2020 -0500 libstdc++: Give ranges::empty() a concrete return type (PR 93978) This works around PR 93978 by avoiding having to instantiate the body of ranges::empty() when checking the constraints of view_interface::operator bool(). When ranges::empty() has an auto return type, then we must instantiate its body in order to determine whether the requires expression { ranges::empty(_M_derived()); } is well-formed. But this means instantiating view_interface::empty() and hence view_interface::_M_derived(), all before we've yet deduced the return type of join_view::end(). (The reason view_interface::operator bool() is needed in join_view::end() in the first place is because in this function we perform direct initialization of join_view::_Sentinel from a join_view, and so we try to find a conversion sequence from the latter to the former that goes through this conversion operator.) Giving ranges::empty() a concrete return type of bool should be safe according to [range.prim.empty]/4 which says "whenever ranges::empty(E) is a valid expression, it has type bool." This fixes the test case in PR 93978 when compiling without -Wall, but with -Wall the test case still fails due to the issue described in PR c++/94038, I think. I still don't quite understand why the test case doesn't fail without -O. libstdc++-v3/ChangeLog: PR libstdc++/93978 * include/bits/range_access.h (__cust_access::_Empty::operator()): Declare return type to be bool instead of auto. * testsuite/std/ranges/adaptors/93978.cc: New test.
