Make std.algo.copy CTFE-able

[DmitryOlshansky]

No description provided.

[quickfur]

Nice! I like it.

Will defer to another reviewer to approve, though. Just in case I missed something important.

[JackStouffer]

Why use the first branch when the second one is also CTFE-able and supposedly faster?

[JackStouffer]

int[] func(int[] source, int[] target) {
    const tlen = target.length;
    const slen = source.length;
    target[0 .. slen] = source[];
    return target[slen .. $];
}

void main()
{
	enum a = func([1, 2, 3], [4, 5, 6, 7]);
	pragma(msg, a);
}

works and gives the right answer

[DmitryOlshansky]

Why use the first branch when the second one is also CTFE-able and supposedly faster?

Because we can't guarantee there is no overlap between the arrays. Secondly I highly doubt that CTFE actually does things such as memcpy which is what the fast path tries to get.