[stdlib] New indexing model: Fix LazyFilterCollection index moving

[natecook1000]

What's in this pull request?

The index moving methods weren't functioning correctly. They didn't trap on advancing past endIndex and in some cases did not terminate at all.

let a = [0, 0, 1, 0, 0, 2, 0, 0, 3, 0, 0]
let b = a.lazy.filter { $0 != 0 }
print(b.count)   // doesn't return

Resolved bug number: n/a

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[moiseev]

Here is the test that starts to fail after your change. Don't get me wrong, it's an improvement, as it used to not terminate at all ;-)

tests.test("LazyFilterCollection") {
  let xs = (0..<3).lazy.filter { $0 % 2 == 0 }
  expectEqual(0, xs[xs.startIndex])
}

The problem, I believe, is that _nextFilteredInPlace originally did not call the formSuccessor if the element under index satisfies the predicate. Now it does. So when the first element of a collection satisfies the predicate, it gets skipped.

The following test might help to find more problems like this:

tests.test("LazyFilterCollection") {
  let xs = (0..<3).lazy.filter { $0 % 2 == 0 }
  checkForwardCollection([0, 2], xs) {
    $0 == $1
  }
}

[natecook1000]

Sorry about that! Fixed now and tested on better inputs.

[moiseev]

I ran the tests locally.
Thanks, Nate!