[Bug c++/99018] New: Comparing address of array element not considered a constant expression in certain contexts

["david at doublewise dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99018

            Bug ID: 99018
           Summary: Comparing address of array element not considered a
                    constant expression in certain contexts
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david at doublewise dot net
  Target Milestone: ---

The following valid translation unit

```
struct s {
        constexpr s() = default;

        constexpr s(s const & other) {
                if (this == &other) {
                }
        }
};

constexpr auto f() {
        s init[2];
        for (auto & element : init) {
                s foo = element;
        }
        return true;
}

static_assert(f());
```

is rejected by gcc with

```
<source>:18:16: error: non-constant condition for static assertion
   18 | static_assert(f());
      |               ~^~
<source>:18:16:   in 'constexpr' expansion of 'f()'
<source>:13:11:   in 'constexpr' expansion of 's((*(const s*)(& element)))'
<source>:5:26: error: '(((const s*)(& foo)) == (((const s*)(& init)) + 1))' is
not a constant expression
    5 |                 if (this == &other) {
      |                     ~~~~~^~~~~~~~~
Compiler returned: 1
```

See it live: https://godbolt.org/z/xG5dv5