https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102286
Bug ID: 102286
Summary: [constexpr] construct_at incorrectly starts union
array lifetime in some cases
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ldalessandro at gmail dot com
Target Milestone: ---
Given the following struct and union,
struct Foo {
constexpr explicit Foo(int) {}
};
union U {
Foo data[1];
constexpr U() {} // no active member
};
In the function foo() below, assignment to u.data[0] correctly starts the
lifetime of the data array by https://eel.is/c++draft/class.union#general-6.
constexpr void foo() {
U u;
u.data[0] = 0; // okay
}
However in the function bar() below, &u.data[0] forms a reference before the
start of the lifetime of the data array and thus std::construct_at cannot be
used to start the lifetime of data.
constexpr void bar() {
U u;
std::construct_at(&u.data[0], 0); // forms reference outside lifetime
}
Currently gcc and msvc accept this invalid code, while clang and icc reject.
See the thread of discussion at
https://lists.isocpp.org/std-proposals/2021/09/3171.php and a live example at
https://godbolt.org/z/Poh6bb5de.
