https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108822
--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:f8a5298c97c460d45e888b123fe1bbcdb49b8ad4 commit r14-7225-gf8a5298c97c460d45e888b123fe1bbcdb49b8ad4 Author: Jonathan Wakely <jwak...@redhat.com> Date: Wed Jan 10 17:29:22 2024 +0000 libstdc++: Implement P2255R2 dangling checks for std::tuple [PR108822] This is the last part of PR libstdc++/108822 implementing P2255R2, which makes it ill-formed to create a std::tuple that would bind a reference to a temporary. The dangling checks are implemented as deleted constructors for C++20 and higher, and as Debug Mode static assertions in the constructor body for older standards. This is similar to the r13-6084-g916ce577ad109b changes for std::pair. As part of this change, I've reimplemented most of std::tuple for C++20, making use of concepts to replace the enable_if constraints, and using conditional explicit to avoid duplicating most constructors. We could use conditional explicit for the C++11 implementation too (with pragmas to disables the -Wc++17-extensions warnings), but that should be done as a stage 1 change for GCC 15 rather than now. The partial specialization for std::tuple<T1, T2> is no longer used for C++20 (or more precisely, for a C++20 compiler that supports concepts and conditional explicit). The additional constructors and assignment operators that take std::pair arguments have been added to the C++20 implementation of the primary template, with sizeof...(_Elements)==2 constraints. This avoids reimplementing all the other constructors in the std::tuple<T1, T2> partial specialization to use concepts. This way we avoid four implementations of every constructor and only have three! (The primary template has an implementation of each constructor for C++11 and another for C++20, and the tuple<T1,T2> specialization has an implementation of each for C++11, so that's three for each constructor.) In order to make the constraints more efficient on the C++20 version of the default constructor I've also added a variable template for the __is_implicitly_default_constructible trait, implemented using concepts. libstdc++-v3/ChangeLog: PR libstdc++/108822 * include/std/tuple (tuple): Add checks for dangling references. Reimplement constraints and constant expressions using C++20 features. * include/std/type_traits [C++20] (__is_implicitly_default_constructible_v): Define. (__is_implicitly_default_constructible): Use variable template. * testsuite/20_util/tuple/dangling_ref.cc: New test. Reviewed-by: Patrick Palka <ppa...@redhat.com>
