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

[Ed Smith-Rowland]

On 11/16/18 12:38 PM, Daniel Krügler wrote:
> 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
>
I did not see that those were not constexpr.  I guess arg needs atan2 
and proj

I'll remove the constexpr on those and adjust everything.

Thank you.

Ed