Test for C++20 p0858 - ConstexprIterator requirements.

[Ville Voutilainen]

On Sat, 1 Jun 2019 at 22:40, Ed Smith-Rowland <3dw4rd@verizon.net> wrote:

> Ok, third time's a charm.
>
> I was brain dead about the constexpr patch.?? I'm now setting a constexpr
> variable from test() in a caller.

Looks good. Jonathan needs to approve it, though.

> But static_assert is a constexpr context no?

Yes, it is. It's just that now all of test() is verified to be okay as
a constant expression, including std::copy.