@dabrahams Could you give some examples of the limitations that you're running into here?

IIRC, the reason that CombinationSequence is only a sequence is that its iterator stores and mutates a single copy of the base collection's indices as it iterates. By contrast, a collection would need to provide an index type that either stores a separate copy of the indices (potentially very heavyweight) or stores just a position, and then recalculates the indices when used (which should be doable, but might be costly). While the Collection protocol doesn't explicitly rule out such a heavy index type, it's very different from the way most collections work, and having O(N) work happening in each subscript would probably be unexpected for users.

That said, CombinationSequence might already be falling down on the efficiency job. It looks like the iterator currently copies each combination into a new array, instead of using a collection wrapper to combine the base collection and the array of indices. Such a wrapper would be independently useful, as well (and indeed I thought we already included it):

let numbers = [10, 20, 30, 40, 50]
for x in numbers.indexed(by: [4, 2, 0]) {
    print(x)
}
// 50
// 30
// 10

CombinationsSequence should be a Collection #219

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`CombinationsSequence` should be a `Collection` #219