The constexpr function f in the test case below has well-defined behavior only when its argument is non-zero. When its argument is zero, the function returns a value obtained by dereferencing a dangling pointer. Since an expression with undefined behavior is not a valid core constant expression it cannot be used to initialize a constexpr variable and the whole program below should be rejected with an error. However, GCC accepts it without a diagnostic. $ cat x.c && /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -S -Wall -Wextra -Wpedantic -o/dev/stdout -xc++ x.c constexpr int f (int i) { int *p = &i; if (i == 0) { int j = 123; p = &j; } return *p; } constexpr int i = f (0); const int j = i; For comparison, Clang produces the following output: x.c:12:15: error: constexpr variable 'i' must be initialized by a constant expression constexpr int i = f (0); ^ ~~~~~ x.c:9:12: note: read of object outside its lifetime is not allowed in a constant expression return *p; ^ x.c:12:19: note: in call to 'f(0)' constexpr int i = f (0); ^ 1 error generated.
Confirmed.
*** Bug 93389 has been marked as a duplicate of this bug. ***
*** Bug 102603 has been marked as a duplicate of this bug. ***
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>