https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108846
--- Comment #35 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-13 branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:84eb2ddc53344259c5b3980d1b5dad69b82bfe8b commit r13-9540-g84eb2ddc53344259c5b3980d1b5dad69b82bfe8b Author: Giuseppe D'Angelo <giuseppe.dang...@kdab.com> Date: Fri Aug 23 15:05:54 2024 +0200 libstdc++: Do not use use memmove for 1-element ranges [PR108846,PR116471] This commit ports the fixes already applied by r13-6372-g822a11a1e642e0 to the range-based versions of copy/move algorithms. When doing so, a further bug (PR116471) was discovered in the implementation of the range-based algorithms: although the algorithms are already constrained by the indirectly_copyable/movable concepts, there was a failing static_assert in the memmove path. This static_assert checked that iterator's value type was assignable by using the is_copy_assignable (move) type traits. However, this is a problem, because the traits are too strict when checking for constness; a type like struct S { S& operator=(S &) = default; }; is trivially copyable (and thus could benefit of the memmove path), but it does not satisfy is_copy_assignable because the operator takes by non-const reference. Now, the reason for the check to be there is because a type with a deleted assignment operator like struct E { E& operator=(const E&) = delete; }; is still trivially copyable, but not assignable. We don't want algorithms like std::ranges::copy to compile because they end up selecting the memmove path, "ignoring" the fact that E isn't even copy assignable. But the static_assert isn't needed here any longer: as noted before, the ranges algorithms already have the appropriate constraints; and even if they didn't, there's now a non-discarded codepath to deal with ranges of length 1 where there is an explicit assignment operation. Therefore, this commit removes it. (In fact, r13-6372-g822a11a1e642e0 removed the same static_assert from the non-ranges algorithms.) libstdc++-v3/ChangeLog: PR libstdc++/108846 PR libstdc++/116471 * include/bits/ranges_algobase.h (__assign_one): New helper function. (__copy_or_move): Remove a spurious static_assert; use __assign_one for memcpyable ranges of length 1. (__copy_or_move_backward): Likewise. * testsuite/25_algorithms/copy/108846.cc: Extend to range-based algorithms, and cover both memcpyable and non-memcpyable cases. * testsuite/25_algorithms/copy_backward/108846.cc: Likewise. * testsuite/25_algorithms/copy_n/108846.cc: Likewise. * testsuite/25_algorithms/move/108846.cc: Likewise. * testsuite/25_algorithms/move_backward/108846.cc: Likewise. Signed-off-by: Giuseppe D'Angelo <giuseppe.dang...@kdab.com> (cherry picked from commit 5938e0681c3907b2771ce6717988416b0ddd2f54)
