[Bug c++/97195] New: construct_at on a union member is not a constant expression

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97195

            Bug ID: 97195
           Summary: construct_at on a union member is not a constant
                    expression
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

In trying to test adding more constexpr support to std::optional:

#include <bits/stl_construct.h>

union S {
    int i;
};

constexpr int f() {
    S s;
    std::construct_at(&s.i, 0);
    return 0;
}

constexpr int z = f();

gcc rejects with:

<source>:13:20:   in 'constexpr' expansion of 'f()'
<source>:9:22:   in 'constexpr' expansion of 'std::construct_at<int, int>((&
s.S::i), 0)'
<source>:13:21: error: '(void*)(& s.S::i)' is not a constant expression
   13 | constexpr int z = f();
      |                     ^

But this should be fine.