Patches to improve compatibility with modern C++ standard

[Richard Kimberly Heck]

On 5/4/20 12:05 AM, Thibaut Cuvelier wrote:
> One of the largest changes is due to the addition of requires to the
> list of keywords, which is currently replaced by requirements, which
> is close in meaning.

Most of these have been fixed, and I think I just fixed the rest.

I don't see a clang warning here with requires_, which I do get for
requires by itself. Are you sure that's needed?

Riki