Friended struct with concept gives different output in clang/gcc/MSVC

[danakj]

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

I don't know which compiler is right, but all three give a different output.

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

[danakj]

This should be labeled for C++20

[llvmbot]

@llvm/issue-subscribers-c-20

[danakj]

The GCC bug mentions https://cplusplus.github.io/CWG/issues/2589.html however it's not clear that caching is the problem in this case. Reordering the uses of the HasTag concept does not change the output.

[frederick-vs-ja]

For Clang (trunk), currently reording two statements can still change the results, which indicates that Clang doesn't implement the proposed resolution of CWG2589, but remembers the first result of the constraint, which can be affected by friend.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Dana Jansens (danakj)

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

I don't know which compiler is right, but all three give a different output.

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