https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116440
--- Comment #7 from GCC 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:6570fa6f2612a4e4ddd2fcfc119369a1a48656e4 commit r15-8044-g6570fa6f2612a4e4ddd2fcfc119369a1a48656e4 Author: Patrick Palka <ppa...@redhat.com> Date: Thu Mar 13 19:55:00 2025 -0400 libstdc++: Work around C++20 tuple<tuple<any>> constraint recursion [PR116440] The type tuple<tuple<any>> is clearly copy/move constructible, but for reasons that are not yet completely understood checking this triggers constraint recursion with our C++20 tuple implementation (but not the C++17 implementation). It turns out this recursion stems from considering the non-template tuple(const _Elements&) constructor during the copy/move constructibility check. Considering this constructor is ultimately redundant, since the defaulted copy/move constructors are better matches. GCC has a non-standard "perfect candidate" optimization[1] that causes overload resolution to shortcut considering template candidates if we find a (non-template) perfect candidate. So to work around this issue (and as a general compile-time optimization) this patch turns the problematic constructor into a template so that GCC doesn't consider it when checking for copy/move constructibility of this tuple type. Changing the template-ness of a constructor can affect overload resolution (since template-ness is a tiebreaker) so there's a risk this change could e.g. introduce overload resolution ambiguities. But the original C++17 implementation has long defined this constructor as a template (in order to constrain it etc), so doing the same thing in the C++20 mode should naturally be quite safe. The testcase still fails with Clang (in C++20 mode) since it doesn't implement said optimization. [1]: See r11-7287-g187d0d5871b1fa and https://isocpp.org/files/papers/P3606R0.html PR libstdc++/116440 libstdc++-v3/ChangeLog: * include/std/tuple (tuple::tuple(const _Elements&...)) [C++20]: Turn into a template. * testsuite/20_util/tuple/116440.C: New test. Reviewed-by: Jonathan Wakely <jwak...@redhat.com>
