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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 17:16:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98216

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:

struct an_array
{
  constexpr bool operator=3D=3D(const an_array& a) const { return d =3D=3D =
a.d; }

  double d;
};

constexpr auto
add_array (an_array a, an_array b)
{
  a.d +=3D b.d;
  return a;
}

template <an_array A>
struct wrapper
{
  static constexpr auto arr =3D A;
};

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 ()
{
#ifndef FIX
  auto a =3D wrapper <MINUS_TWO> {};
  __builtin_printf("%f\n", a.arr.d);
#endif
  auto c =3D wrapper <add_array (MINUS_TWO, ONE)> {};
  __builtin_printf("%f\n", c.arr.d);
  if (c.arr !=3D MINUS_ONE)
    __builtin_abort();
}

Prints:

-2.000000
-2.000000
Aborted (core dumped)

If you define FIX (i.e. don't instantiate wrapper<MINUS_TWO> first) it work=
s:

-1.000000=