In 2016 I asked this question:
it was closed as lacking details or clarity. The comment was:
Need details of which platform, CPU, OS, etc., to possibly answer.
I explained why a constexpr header-only library is by definition irrespective of these details; and I also edited to clarify that the library must be fully cross-platform (which any constexpr functionality library would have to be actually). Other commenters also chimed in, saying that the question was perfectly clear.
Still, 4 years have passed and the question has not been reopened. I have answers I wish to add - but cannot.