Why does this VC++ workaround not work in GCC or clang?

I came across this interesting article about how to implement a compile time counter in C++.

The author provides a workaround for Visual C++. I slightly modified it and compiled it online and it indeed works when compiled with Visual C++.

#include <iostream>

template<int N>
struct flag {
  friend constexpr int adl_flag (flag<N>);
};

template<int N>
struct writer {
  friend constexpr int adl_flag (flag<N>) {
    return N;
  }

  static constexpr int value = N;
};

template<int N, class = char[noexcept(adl_flag(flag<N> ()))?+1:-1]>
int constexpr reader (int, flag<N>) {
  return N;
}

template<int N>
int constexpr reader (float, flag<N>, int R = reader (0, flag<N-1> ())) {
  return R;
}

int constexpr reader (float, flag<0>) {
  return 0;
}

template<int N = 1, int C = reader (0, flag<32> ())>
int constexpr next (int R = writer<C + N>::value) {
  return R;
}


int main () {
  constexpr int a = next();
  constexpr int b = next();
  constexpr int c = next();

  std::cout << a << " " << b  << " " << c << std::endl;
  static_assert (a == 1 && b == a+1 && c == b+1, "try again");
}

output with VC++

1 2 3

However, the assertation in that code fails for both GCC and clang. When commenting out the assertion, the output is:

1 1 1

Is this workaround exploiting a bug in VC++ or should it work in GCC/clang as well?