[Bug c++/87381] clang 6.0 will compile this constexpr construct, but gcc 8.2.1 will not.

[eric-bugs at omnifarious dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87381

--- Comment #1 from eric-bugs at omnifarious dot org ---
Godbolt link: https://godbolt.org/z/gHnb-G

Also, my attempt to simplify this failed because clang will not consider
arguments to constexpr functions to be constexpr. Which, IMHO, is wrong.
Whether the fault is in the standard or clang, I don't know.

I didn't test to see if my attempts to simplify it would work in gcc because
even this non-simplified example fails.