Some constexpr code fails to compile on GCC < 7.2

[Ghabry]

Some of our code appears to fail to compile with older GCC as noticed by someone on Twitter:

See

• https://twitter.com/_imai/status/1271352248115683329
• http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1684
• https://pastebin.com/29RUdk99
• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66297

Maybe this worked in gcc5 but broke in 5.1 - 7.1?

[fmatthew5876]

This is a bug anyway, those functions should be static. I fixed it in my huge PR, but I can backport this small fix to unblock people.

[fmatthew5876]

Also would it make sense / be a lot of work to setup a new linux / gcc5 build? Basically a jenkins build that only runs on merge master which tests our advertised minimum supported compiler version.