struct X { int i = 0; constexpr int& get() { return i; } }; constexpr int g() { const auto& i = X{}.get(); return i; // undefined } constexpr auto i = g(); G++ fails to diagnose the error here. Clang says: <source>:13:16: error: constexpr variable 'i' must be initialized by a constant expression constexpr auto i = g(); ^ ~~~ <source>:10:12: note: read of object outside its lifetime is not allowed in a constant expression return i; // undefined ^ <source>:13:20: note: in call to 'g()' constexpr auto i = g(); ^ Intel says: <source>(13): error: expression must have a constant value constexpr auto i = g(); ^ <source>(10): note: attempt to access expired storage return i; // undefined ^
Confirmed.
Here's another testcase (derived from code in the Eigen library ) that seems to illustrate the same issue. Please follow the compiler explorer link to see a non-constexpr version and its gimple that makes clear that there indeed is a lifetime issue and also clang's error message https://godbolt.org/z/4zoKaoKa5 struct B; #define CONSTEXPR constexpr #define CONSTEVAL consteval struct A { public: CONSTEXPR A() { m_val = 1; } CONSTEXPR A(const A& other) { m_val = other.m_val;} CONSTEXPR ~A() {}; CONSTEXPR int val() { return m_val; } int m_val; CONSTEXPR B operator<<(int); }; struct B { A& m_a; CONSTEXPR B(A& ref) : m_a(ref) {} CONSTEXPR B& operator,(int i) { m_a.m_val = i; return *this; } CONSTEXPR ~B() { finished(); } CONSTEXPR A finished() { return m_a; } }; CONSTEXPR B A::operator<<(int i) { m_val = i; return B(*this); } CONSTEVAL int f() { A a = (A() << 1, 2, 3, 4).finished(); return a.val(); } int g() { return f(); }
Sorry, in my example, I think actually clang is wrong.
(In reply to Tobias Schlüter from comment #3) > Sorry, in my example, I think actually clang is wrong. What is the order of destruction of tempories here in the following statement: A() << 1 Is A() destoryed before the temp B that is returned from operator <<. Note MSVC accepts the code so it might be a bug in clang.
I've submitted this to clang's bug tracker as well. https://github.com/llvm/llvm-project/issues/53494
*** Bug 109991 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>