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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/98216] [C++20] std::array<double, N> template parameter
 error with negative values
Date: Thu, 10 Dec 2020 17:05:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98216

--- 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=3D=3D(const array& a) const
  {
    for (int i =3D 0; i < N; ++i)
      if (d[i] !=3D a.d[i])
        return false;
    return true;
  }

  T d[N];
};

using an_array =3D array <double, 1>;

constexpr auto
add_array (an_array a, an_array const& b)
{
  a[0] +=3D b[0];
  return a;
}

template <an_array A>
struct wrapper
{
  static constexpr auto
    arr =3D 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       =3D an_array {{  1. }};
constexpr auto MINUS_ONE =3D an_array {{ -1. }};=20
constexpr auto MINUS_TWO =3D an_array {{ -2. }};=20

  int
main ()
{
  auto a =3D wrapper <MINUS_TWO> {};
  auto b =3D wrapper <ONE> {};
  auto c =3D add (a, b);
  __builtin_printf("%f %f %f\n", a.arr[0], b.arr[0], c.arr[0]);
  if (c.arr !=3D MINUS_ONE)
    __builtin_abort();
}

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