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UB, unions, common initial sequence and constexpr

From: Wilhelm Meier <wilhelm.meier_at_[hidden]>
Date: Fri, 21 Oct 2022 13:36:57 +0200
HI all,

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

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?

Received on 2022-10-21 11:36:59