[Bug c++/98216] [C++20] std::array template parameter error with negative values

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98216

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:

template<typename T, int N>
struct array
{
  constexpr T& operator[](int n) { return d[n]; }
  constexpr const T& operator[](int n) const { return d[n]; }

  constexpr bool operator==(const array& a) const
  {
    for (int i = 0; i < N; ++i)
      if (d[i] != a.d[i])
        return false;
    return true;
  }

  T d[N];
};

using an_array = array <double, 1>;

constexpr auto
add_array (an_array a, an_array const& b)
{
  a[0] += b[0];
  return a;
}

template <an_array A>
struct wrapper
{
  static constexpr auto
    arr = A;
};


template <an_array A1 , an_array A2>
auto
add (wrapper <A1> const&, wrapper <A2> const&)
{
  return wrapper <add_array (A1, A2)> {};
}

constexpr auto ONE       = an_array {{  1. }};
constexpr auto MINUS_ONE = an_array {{ -1. }}; 
constexpr auto MINUS_TWO = an_array {{ -2. }}; 

  int
main ()
{
  auto a = wrapper <MINUS_TWO> {};
  auto b = wrapper <ONE> {};
  auto c = add (a, b);
  __builtin_printf("%f %f %f\n", a.arr[0], b.arr[0], c.arr[0]);
  if (c.arr != MINUS_ONE)
    __builtin_abort();
}

Prints:
-2.000000 1.000000 -2.000000
Aborted (core dumped)