[Bug c++/107544] New: Friended struct with concept gives different output in clang/gcc/MSVC

[danakj at orodu dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107544

            Bug ID: 107544
           Summary: Friended struct with concept gives different output in
                    clang/gcc/MSVC
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: danakj at orodu dot net
  Target Milestone: ---

I don't know which compiler is right, but all three give a different output. So
I will report it here, as the GCC result looks wrong and exposes private data.

https://godbolt.org/z/7Pq3eWhc8

```
#include <compare>
#include <concepts>
#include <iostream>

template <class T>
concept HasTag = requires {
    T::Tag;
    requires std::same_as<decltype(T::Tag), const bool>;
};

template <class T>
struct check_tag final {
    static constexpr bool value()
        requires(!HasTag<T>)
    {
        return false;
    }

    static constexpr bool value()
        requires(HasTag<T>)
    {
        return T::Tag;
    };
};

struct S {
   private:
    template <class T>
    friend struct check_tag;
    static constexpr bool Tag = true;
};

int main() {
    std::cout << HasTag<S> << "\n";
    std::cout << check_tag<S>::value() << "\n";
}
```

Clang output (the concept sees private data if used in friend):
0
1

GCC output (the concept always sees private data):
1
1

MSVC output (the concept never sees private data):
0
0