If we can get a consensus on support for GCC's libquadmath in Boost.Multiprecision, I would volunteer to write the backend for it. But I need a few months before I get that kind of time slot for writing, testing, etc. I wonder what John and Paul think about adding support for this. I'm skeptical because the type really only works with one compiler, and then you need to --enable-quadmath when building GCC in order to get it. It's a nightmare!
I think this is really the issue - figuring out when to enable support for this feature. BTW, I can see a genuine use case for constexpr 128-bit float constants even if the right way to handle __float128 is to wrap it in the Multiprecision lib. I wonder are there any predefined macros we can use like __SIZEOF_INT128__ is for __int128? John.