[Bug c++/111423] New: bogus Wsubobject-linkage when using CRTP with an invisible base, template template parameters, and concepts

[brunopitrus at hotmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111423

            Bug ID: 111423
           Summary: bogus Wsubobject-linkage when using CRTP with an
                    invisible base, template template parameters, and
                    concepts
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: brunopitrus at hotmail dot com
  Target Milestone: ---

header.h:
```
template <typename T>
concept bar = requires(const T& t) {
  { t.GetA() };
};

template <template<typename> typename T>
struct Derived : T<Derived<T>>{};
```

main.cpp:
```
#include "header.h"

namespace {
template<typename Derived>
struct Base2 {
    void GetA() const {}
};

using B2 = Derived<Base2>;

static_assert(bar<B2>);
}
```

This code makes GCC complain with
```
In file included from /app/main.cpp:1:
header.h: In instantiation of 'struct Derived<{anonymous}::Base2>':
/app/header.h:3:7:   required from here
/app/header.h:7:8: error: 'Derived<{anonymous}::Base2>' has a base
'{anonymous}::Base2<Derived<{anonymous}::Base2> >' which uses the anonymous
namespace [-Werror=subobject-linkage]
    7 | struct Derived : T<Derived<T>>{};
      |        ^~~~~~~
```

which is incorrect — `Derived<{anonymous}::Base2>` should have internal linkage
due to it being dependent on the internal-linkage `{anonymous}::Base2` —
therefore it should not matter what linkage its bases have.

See it on Godbolt:
https://godbolt.org/z/q7jb5bGb6