Array as non-type template argument in C++17

nontype.cc

#include "nontype.h"

const char tag[] = "abc";

int main()
{
}

nontype.h

template <const char* T>
struct X {
};

extern const char tag[];
typedef X<tag> Y;

// $ clang++ nontype.cc -std=c++1z
// nontype.h:6:11: error: non-type template argument refers to subobject '&tag'
// typedef X<tag> Y;
//           ^
// 1 error generated.

const char tag2[] = "abc";
struct Z {
  X<tag2> x_;
};

// $ g++ nontype.cc -c -std=c++17 
// nontype.h:15:8: warning: 'Z' has a field 'Z::x_' whose type uses the anonymous namespace [-Wsubobject-linkage]
// struct Z {
//        ^