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From: "emmanuel.le-trong@cnrs-orleans.fr" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/98216] New: [C++20] std::array<double, N> template
 parameter error with negative values
Date: Wed, 09 Dec 2020 15:43:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98216

            Bug ID: 98216
           Summary: [C++20] std::array<double, N> template parameter error
                    with negative values
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: emmanuel.le-trong@cnrs-orleans.fr
  Target Milestone: ---

Sorry for the bad title, I can't figure out how to summarize this:

    #include <array>
    #include <cassert>

        using
    an_array =3D std::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);
        assert (c.arr =3D=3D MINUS_TWO); // <- should fail
        assert (c.arr =3D=3D MINUS_ONE); // <- should pass
    }

Compiled with version 11.0.0 20201209, the first assertion passes and the
second fails. It should be the opposite.

FWIW, if you replace double by float, it works as intended.=