Fixing the SortedRange

[Dmitry Olshansky]

It's annoyingly strange that e.g. the following won't work as of yet:

auto sorted = assumeSorted(["hello", "world"]); //assume lots of strings
auto x = sorted.lowerBound("Try that?"d);

Yeah, I know the immutable(dchar)[] vs immutable(char)[] can't be 
compared using straight <. That's acceptable, even if not convenient.

Ok, take 2, via std.algorithm cmp:
auto sorted = assumeSorted!"cmp(a,b) < 0"(["hello", "world"]);
auto x = sorted.lowerBound("Try that?"d);

Now that fails, because of lowerBound signature. And that is

auto lowerBound(SearchPolicy sp = SearchPolicy.binarySearch, V)(V value)
     if (is(V : ElementType!Range))

Same goes for upperBound and others.

And why the hell I need to convert the value to type of range element? 
It's not like there is any value = range[x]; necessary, and indeed it 
works without the signature (+ a one liner fix).

Seeking the proper signature I've come up with the following:

auto lowerBound(SearchPolicy sp = SearchPolicy.binarySearch, V)(V value)
     if (isTwoWayCompatible!(predFun, ElementType!Range, V))

isTwoWayCompatible!(pred,A,B) means ether of the following works:
pred(a,b) and pred(b,a) where a has type A, b typed as B

Seem fine to me, so I'm looking for a better name for this trait.
Does it sound like it belongs in std.traits or is there more general 
better abstraction for this kind of thing?


-- 
Dmitry Olshansky