On 21 October 2022 12:36:57 BST, Wilhelm Meier via Std-Discussion <std-discussion@lists.isocpp.org> wrote:

HI all,

the standard explicitly makes this exception: accessing an inactive union member is not UB if the members are structs with initial common sequence.

Is this also true in a constexpr context? I ask this because the chapter for constexpr expression does not mention this exception accessing an inactive memeber.

Here is some test-code:

struct A {
    char e0;
};
struct B {
    char e0;
};

template<typename A, typename B>
union U {
    A a;
    B b;
};

int main() {
    constexpr U<A, B> u{.a = {1}}; // sets member a active
    constexpr char t1 = u.b.e0; // should be ok, because read via a struct with common initial sequence, but also in a constexpr context?
}
-- 
Wilhelm