Hello,

When you assign to b[0] a is no longer the active member of the union
and therefore reading from it is undefined behavior, we can see this
from the diagnostic in this example program:
https://godbolt.org/z/1PjfzjEf8


On Thu, Mar 16, 2023 at 1:20 PM Stephan Bergmann via Std-Discussion
<std-discussion_at_[hidden]> wrote:
>
> I'm trying to understand what the semantics of the C++20 program
>
> > struct S {
> > union {
> > char a[2];
> > char b[1];
> > };
> > constexpr S() {
> > a[1] = 2;
> > b[0] = 3;
> > }
> > };
> > constexpr S s;
> > int main() {
> > return reinterpret_cast<unsigned char const *>(&s)[1];
> > }
>
> is, but I fail to find anything relevant in the standard. Plausible
> results could be UB, "return 0" (which is what at least Clang and GCC
> appear to agree on), or even "return 2". Any pointers?
>
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