fix Issue 7300 - std.regex.ShiftOr!dchar.search is broken

[DmitryOlshansky]

Aside from fixing this particular bug in one codepath of ShiftOr this includes

• A start on cleaning up old CTFE and compiler workarounds.
• Caching regexes was part of original std.regex though it had single recently used slot.

[DmitryOlshansky]

I'm rebasing this. Ehm kills code comments :(

[DmitryOlshansky]

Anything else on this pull? I'd really like to move this forward.

[DmitryOlshansky]

Extended, fixed and rebased.

[dnadlinger]

Looks fine to me – I wonder, though, if isTwowayCompatible should be publicly documented, so that users can make sense of failing template constraints.

[DmitryOlshansky]

If documented then we are looking for a better name, something like isTwowayOrder or isTwowayComparator, or whatever.

[DmitryOlshansky]

Ok, I've settled on just documenting isTwoWayCompatible. Any good ideas for renaming it are still welcome.

[tgehr]

The documentation is inconsistent with the example, and both are inconsistent with the implementation.

[DmitryOlshansky]

I think the example is what documentation states.
Yet, the actual test code is even more restricted.
I'll make a pull to get this in line.

[tgehr]

The example also requires that T1 and T2 are default-constructible. You could initialize them with void to fix this.

[DmitryOlshansky]

The trick is ... it doesn't compile with:
T1 e=void;
T2 v=void;
fn(v,e);
fn(e,v);

I believe it has something to do with string predicates.
Like compiler checking "value used before initialized", it works ok for => predicates.

Problem happens with code like this:
import std.range;

void main(){
int[] abc = [1, 2, 3];
auto srange = assumeSorted!"<="(abc);
srange.lowerBound(2);
}

[tgehr]

What about

T1 foo();
T2 bar();

fn(foo(), bar());
fn(bar(),foo());

?

[DmitryOlshansky]

Nice one.
#521