P0226R1: Mathematical Special Functions for C++17

[cjdb]

Link: https://wg21.link/p0226r1

Subtasks:

• sf.cmath.assoc.laguerre std::assoc_laguerre
• sf.cmath.assoc.legendre std::assoc_legendre
• sf.cmath.beta std::beta
• sf.cmath.comp.ellint.1 std::comp_ellint_1
• sf.cmath.comp.ellint.2 std::comp_ellint_2
• sf.cmath.comp.ellint.3 std::comp_ellint_3
• sf.cmath.cyl.bessel.i std::cyl_bessel_i
• sf.cmath.cyl.bessel.j std::cyl_bessel_j
• sf.cmath.cyl.bessel.k std::cyl_bessel_k
• sf.cmath.cyl.neumann std::cyl_neumann
• sf.cmath.ellint.1 std::ellint_1
• sf.cmath.ellint.2 std::ellint_2
• sf.cmath.ellint.3 std::ellint_3
• sf.cmath.expint std::expint
• sf.cmath.hermite std::hermite (PR)
• sf.cmath.laguerre std::laguerre
• sf.cmath.legendre std::legendre
• sf.cmath.riemann.zeta std::riemann_zeta
• sf.cmath.sph.bessel std::sph_bessel
• sf.cmath.sph.legendre std::sph_legendre
• sf.cmath.sph.neumann std::sph_neumann

BEGIN-RST-NOTES
Progress is tracked `here <https://github.com/llvm/llvm-project/issues/99939>`__
END-RST-NOTES

[llvmbot]

@llvm/issue-subscribers-c-17

Author: Christopher Di Bella (cjdb)

[h-vetinari]

There were several attempts at this (not claiming that the list below is exhaustive):

• https://reviews.llvm.org/D58876
• https://reviews.llvm.org/D59937
• https://reviews.llvm.org/D142806

The last one is interesting for just using boost outright (like STL does), but that's in limbo with the LLVM board(?) around license questions. There was also this plan in some libc++ meeting notes from last year given in #72055 (there are probably several issues).

[ldionne]

There actually has been progress on the LLVM Board side of things since then. We would be ready to make an official request for using Boost as a back-end to the Board, the framework should be in place to treat this request now. CC @llvm-beanz

[mordante]

@PaulXiCao is working on this. The first function will be shipped in libc++ 19 :-)

[PaulXiCao]

Including Boost.Math sounds like a really great approach! I was told the licensing topic is too complex for a short-term solution which is why I started implementing them on my own.

I started out from the above linked, abandoned mr (from legacy Phabricator). The first function that got merged just recently was std::hermite (this mr for reference).
I started work on std::legendre, std::assoc_legendre, and std::laguerre some time ago. I planned to continue now that std::hermite got merged.

On the other hand, it might be more reasonable to wait for a final decision on the usage of Boost.Math?