Created attachment 49065 [details] Example code This program uses a reference to an out-of-scope local variable in constexpr context. struct ii { int v; constexpr const int& get() const { return v;} constexpr ii& inc() { ++v; return *this;} }; struct wrap { ii i; constexpr const int& getnext() const { auto copy = i; return copy.inc().get();} }; template <int> struct S {}; S<wrap{{3}}.getnext()> s; It is, however, accepted and compiles. If used in runtime evaluation, the undefined behaviour causes wildly different results depending on optimization level. bf :-) confuciusornis /tmp> g++-10 --version g++-10 (Ubuntu 10-20200411-0ubuntu1) 10.0.1 20200411 (experimental) [master revision bb87d5cc77d:75961caccb7:f883c46b4877f637e0fa5025b4d6b5c9040ec566] Copyright (C) 2020 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. bf :-) confuciusornis /tmp> g++-10 -std=c++20 -c -Wall -Wextra -pedantic t.cpp bf :-) confuciusornis /tmp> Godbolt link: https://godbolt.org/z/8q91dv
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>: https://gcc.gnu.org/g:9fdbd7d6fa5e0a76898dd66658934e3184111680 commit r14-2773-g9fdbd7d6fa5e0a76898dd66658934e3184111680 Author: Nathaniel Shead <nathanieloshead@gmail.com> Date: Sun Jul 23 01:15:14 2023 +1000 c++: Track lifetimes in constant evaluation [PR70331,PR96630,PR98675] This adds rudimentary lifetime tracking in C++ constexpr contexts, allowing the compiler to report errors with using values after their backing has gone out of scope. We don't yet handle other ways of accessing values outside their lifetime (e.g. following explicit destructor calls). PR c++/96630 PR c++/98675 PR c++/70331 gcc/cp/ChangeLog: * constexpr.cc (constexpr_global_ctx::is_outside_lifetime): New function. (constexpr_global_ctx::get_value): Don't return expired values. (constexpr_global_ctx::get_value_ptr): Likewise. (constexpr_global_ctx::remove_value): Mark value outside lifetime. (outside_lifetime_error): New function. (cxx_eval_call_expression): No longer track save_exprs. (cxx_eval_loop_expr): Likewise. (cxx_eval_constant_expression): Add checks for outside lifetime values. Remove local variables at end of bind exprs, and temporaries after cleanup points. gcc/testsuite/ChangeLog: * g++.dg/cpp1y/constexpr-lifetime1.C: New test. * g++.dg/cpp1y/constexpr-lifetime2.C: New test. * g++.dg/cpp1y/constexpr-lifetime3.C: New test. * g++.dg/cpp1y/constexpr-lifetime4.C: New test. * g++.dg/cpp1y/constexpr-lifetime5.C: New test. * g++.dg/cpp1y/constexpr-lifetime6.C: New test. Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>