[Bug c++/97664] New: constexpr evaluation incorrectly accepts non-constant destruction

[ldalessandro at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97664

            Bug ID: 97664
           Summary: constexpr evaluation incorrectly accepts non-constant
                    destruction
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldalessandro at gmail dot com
  Target Milestone: ---

This one involves some language-lawyering, which is not my forte.

While developing a constexpr, array-based vector that supports non-default
constructible types, I have run into the following inconsistency between clang
and gcc. 

https://godbolt.org/z/PYdq5P

```
#include <memory>

struct Foo {
    mutable int n = 0;
};

union U {
    Foo foo;
    constexpr U() {}
};

struct cvector {
    U storage[1];
    constexpr cvector() {
        std::construct_at(&storage[0].foo);
    }
    constexpr ~cvector() {
        std::destroy_at(&storage[0].foo);
    }
};

constexpr cvector v;
```

After some slack discussion that I didn't entirely follow, it seems that
http://eel.is/c++draft/expr.const#7.2 combined with
http://eel.is/c++draft/expr.const#6.2 may exclude the constexpr use of
destroy_at of an instance of a class with mutable members. I am not confident
with this analysis, nor am I confident that this is desirable behavior, but am
reporting it for consideration.