[PATCH, libstdc++] Implement P0415 More constexpr for std::complex.

[Daniel Krügler]

Am Fr., 16. Nov. 2018 um 18:13 Uhr schrieb Ed Smith-Rowland
<3dw4rd@verizon.net>:
>
> Greetings,
>
> This is late but I wanted to put it out there just to finish a thing.
>
> It's fairly straightforward constexpr of operators and some simple
> functions for std::complex.
>
> The only thing that jumped out was the norm function.  We had this:
>
>      struct _Norm_helper<true>
>      {
>        template<typename _Tp>
>          static inline _Tp _S_do_it(const complex<_Tp>& __z)
>          {
>            _Tp __res = std::abs(__z);
>            return __res * __res;
>          }
>      };
>
> Since abs can't be made constexpr for complex since it involves sqrt (It
> probably could but that's another story) I had to fall back to the x^2 +
> y^2.  I don't know who that will bother.  This version should be faster
> and I can't think of any useful trustworthy difference numerically
> either in terms of accuracy of stability.
>
> Barring any feedback on that I'll clean it up and maybe rename my tests
> from constexpr_all_the_things.cc to more_constexpr.cc ;-)
>
> It builds and tests cleanly on x86_64-linux.

Hmmh, according to the recent working draft the following complex
functions are *not* constexpr:

arg, proj

So, shouldn't their new C++20-constexpr specifiers be added
conditionally (In the sense of gcc extensions)?

- Daniel