Making a class template declared in an outer anonymous namespace a friend-CodePudding

Clang refuses to compile the following code (godbolt) while gcc sees no problem with it. Clang error message is shown below line marked (2):

namespace  // Overall anonymous namespace is required and cannot be removed.
{

template <typename T>
struct Friend;

namespace ns
{

class Secret
{
    template <typename T>
    friend struct Friend;     // (1)

    static void foo() {}
};

} // namespace ns

template <typename T>
struct Friend
{
    void bar()
    {
        ns::Secret::foo();   // (2)
        // Error at line (2):
        // 'foo' is a private member of '(anonymous namespace)::ns::Secret'
    }
};

} // anonymous namespace

The reason, I assume, is that line (1) is treated as a declaration of a new class template struct ns::Friend<T> rather than a reference to ::(anonymous namespace)::Friend<T>.

The questions are:

Who's right -- clang or gcc?
If clang is right, then is there a way to make it understand that line (1) doesn't introduce new class template, but refers to existing one?

CodePudding user response：

Yes, you are right that simply friend struct Friend is a declaration of a templated class ::(anonymous namespace)::ns::Friend. Both compilers are right, as when you attempt to use Friend<T>::bar() gcc will complain about access as well (clang just checks a lot earlier than gcc)

To specify the class template in the global namespace, you must write that out:

    template <typename T>
    friend struct ::Friend;     // (1)

This works for GCC, but clang seems broken and doesn't find Friend in the global namespace.

These two workarounds seem to work:

// Make the entire unnamed namespace an inline namespace
inline namespace
{
// ...
}


// Add an explicit using directive to help clang find Friend in `::`
namespace {

template<typename T>
struct Friend;

}
using ::Friend;
namespace {
namespace ns {
// ...
}
}