> 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?

`s` could be just `int s`, and it wouldn't change much, this question is just about «accessing object representation».