Boost: special functions are unavailable on ppc64el

[tamiko]

See [1]:

/usr/include/boost/math/tools/promotion.hpp:267:27: error: static assertion failed: Sorry, but this platform does not have sufficient long double support for the special functions to be reliably implemented.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1059349

[tamiko]

In file included from /usr/include/boost/math/special_functions/detail/round_fwd.hpp:12,
                 from /usr/include/boost/math/special_functions/math_fwd.hpp:29,
                 from /usr/include/boost/math/special_functions/legendre.hpp:16,
                 from /<<PKGBUILDDIR>>/include/deal.II/base/std_cxx17/cmath.h:32,
                 from /<<PKGBUILDDIR>>/source/base/function_lib.cc:20:

Judging from cmath.h the easiest course of action would be to simply override DEAL_II_HAVE_CXX17_BESSEL_FUNCTIONS and DEAL_II_HAVE_CXX17_LEGENDRE_FUNCTIONS to true and live with the fact that the functions are unavailable on ppc64el.

Opinions?

[bangerth]

We don't actually need the long double overload. The only place where we use the legendre function is in fe_series_legendre.cc where we use it with a double argument. The right approach might be to just remove the non-C++17 legendrel() function in our own cmath.h. I would be greatly surprised if anyone used it.

[bangerth]

Separately, though -- in the current dev sources, we're using C++17. How come we fall back to the BOOST versions? Is this with a compiler that doesn't have these implemented, even though they claim to support C++17?

[masterleinad]

https://en.cppreference.com/w/cpp/17 shows that support for mathematical special functions is pretty limited.
You either need

• libstdc++ >= 7, or
• MSVC STL >= 19.14 (VS 2017)

E.g., libc++ hasn't implement them yet.

[masterleinad]

I agree with just removing the long double overloads.

[tamiko]

Roger. Let's just remove the long double overloads. I'll also prepare a patch for 9.5.1 and update the debian package.