[3.1] Remove erroneous drop(while:) optimization

[CodaFi]

Explanation: Calling drop(while:) after prefix() loses the prefix because the internal drop(while:) override grabs the underlying base iterator from _PrefixSequence and wraps it in a _DropWhileSequence. Instead, fall back to Sequence's implementation.

Scope: Affects consumers of the standard library that attempt to chain Sequence transformations that factor through _PrefixSequence.

Reviewer: @airspeedswift

SR Issue: SR-4240

Risk: Very low risk; As this is an optimization, removing this causes a fallback to the correct implementation.

Testing: Regression test included in patch.

(cherry picked from commit 81968e2)

[CodaFi]

@swift-ci please test

[dabrahams]

@swift-ci Please benchmark

[swift-ci]

Build comment file:

Build failed before running benchmark.

---

[palimondo]

That OS X Build failed due to missing benchmark base, if I understand the log correctly?

[CodaFi]

I don't imagine we have any benchmarks that would measure the impact of this change either - and even if we did they would be benchmarking an incorrect implementation.

[dabrahams]

@CodaFi Sorry, but I don't understand what this is supposed to fix. The test, at least, already passes for me without having merged your change.

[CodaFi]

Strange. If I run this in the REPL that ships with beta 4 (Swift 3.1, swiftlang-802.0.42.1, clang-802.0.36), I get this:

let xs = sequence(first: 1, next: {$0 < 10 ? $0 + 1 : nil})

xs: UnfoldSequence<Int, (Int?, Bool)> = { ...

Array(xs.prefix(3).drop(while: {$0 < 7})).isEmpty

$R0: Bool = false

[dabrahams]

@CodaFi Sorry, I was not reading carefully enough. I see this has already been merged to master.

[dabrahams]

@swift-ci Please benchmark

[dabrahams]

A better test would be:

expectEqualSequence([], Array(xs.prefix(3).drop(while: {$0 < 7})))

because if the test failed, you'd get some useful information out of it.

[CodaFi]

I can prepare a patch for master that does this. Do you want this to also land with this PR in 3.1 or does the current PR work on its own?

[swift-ci]

Build comment file:

Build failed before running benchmark.

---

[CodaFi]

@swift-ci please test

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - 9303255f7d44a936b962b78681b227e303422426
Test requested by - @CodaFi

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - 9303255f7d44a936b962b78681b227e303422426
Test requested by - @CodaFi

[palimondo]

Will this land in 3.1 before final version of Xcode 8.3 ships?

[CodaFi]

@dabrahams @airspeedswift Did this just miss the boat?