Find indexes of elements matching a given value in an array

[timotheecour]

Here's a modification to:
1) hide the intermediate struct (as usual in std.algorithm, I 
forgot what this trick is called)
2) work with ranges, not just arrays (ie will work with iota, see 
unittest)
3) accept input without "in" attribute;
4) accept arbitrary predicate, not just "==x"

Please comment if I'm doing anything not casher.

5) In particular, is there a way to avoid the dummy argument in 
this(int ignore)?
6) for (3), I didn't use "in": I believe that is the usual 
behavior in std.algorithm, where the caller is responsible to 
pass in items.dup or items.save if he so wishes. Is that true?
7) I had to remove nothrow and pure, is that needed in the 
general range case? how to fix it if it is?

----
unittest{
    assert(iota(10).findIndexes!`a%3==1`==[1,4,7]);
}
auto findIndexes(alias pred,R)(R items)  if(isInputRange!R) {
	import std.functional:unaryFun;
	struct FindIndexes(alias pred2) {
		private size_t _front;
		this(int ignore=0) {//TODO:this() not allowed so... anything 
better?
			while(!items.empty){
				if(pred2(items.front)) break;
				_front++;
				items.popFront;
			}
		}
		@property bool empty() const {
			return items.empty;
		}
		@property size_t front() const {
			return _front;
		}
		void popFront() {
			_front++;
			items.popFront;
			while(!items.empty){
				if(pred2(items.front)) break;
				_front++;
				items.popFront;
			}
		}
	}
	return FindIndexes!(unaryFun!pred)();
}
----