More constexpr for <cmath> and <complex>

[mborland]

It looks like the following paper has been accepted: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p1383r2.pdf. Since others depend on our special functions in their standard libraries we should probably implement this.

[jzmaddock]

I don't think anyone is using our versions of the functions listed in the paper, but we should probably experiment with this anyway: both for our versions of things listed here (expm1, log1p, erf, tgamma etc) and for other relatively trivial utility functions. Possibly anything that has a simple rational approximation actually.

I have two questions for which I confess I've forgotten the answers:

1. If a function is declared constexpr and calls say a non-constexpr version of pow, can a compiler decline to compile it even if not used in a constexpr context?
2. Even if we're OK on (1), can a compilers optimizer say "Oh goody, a constexpr function, lets optimize that away" even though the context isn't formally constexpr?

If either are true then we will need some annoying configuration logic and macro-ization.

I'm hoping not though... thinking about it, constexpr template functions which are only constexpr depending on certain template arguments would otherwise be impossible?

[ckormanyos]

I've speculated a bit on this in the past times.

I think in C++26 basically most (or all) of the <cmath> functions are supposed to go constexpr. We need someone more dialed into LEWG to really confirm this.

So, since tgamma is present in Math's specfun, it should theoretically go constexpr for C++26. But then when that is the case all of <cmath> should, at the time of that also be `constexpr´.

I do not think this applies to those more advanced functions that used to be in TR1.

But I guess for POD, it might make sense to get a constexpr version of tgamma and any other specfun functions that overlap with those found direclty in <cmath>.

[ckormanyos]

I guess this might be something that finally decides over keeping/not-keeping acosh, asinh and atanh in Math. These, I believe, might also need to go constexpr.