[Bug c++/126278] New: constexpr cast to void* is overly permissive

[ryan.keane051 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126278

            Bug ID: 126278
           Summary: constexpr cast to void* is overly permissive
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ryan.keane051 at gmail dot com
  Target Milestone: ---

```
struct A {};

struct B {};

struct C : B, A {};

static_assert([] {
    C c{};

    void* v = &c;
    auto* b = static_cast<B*>(v);

    return true;
}());
```

constexpr cast to void* should only allow casting directly back to C, but this
code currently compiles on Compiler Explorer. For reference, clang-trunk says:

cast from 'void *' is not allowed in a constant expression because the pointed
object type 'C' is not similar to the target type 'B'
   11 |     auto* b = static_cast<B*>(v);

https://godbolt.org/z/Wanvjq3zd