[SR-4240] drop(while:) after prefix() on pure Sequence looses the prefix

[palimondo]

Previous ID	SR-4240
Radar	None
Original Reporter	@palimondo
Type	Bug
Status	Closed
Resolution	Done

Environment

Swift 3.1beta

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug, 3.1Regression
Assignee	@CodaFi
Priority	Medium

md5: cee58ba90aa08a68ae18b33517c07038

Issue Description:

Looking at the source code, I believe the #4346 introduced a bug in stdlib/public/core/Sequence.swift, where the calling drop(while: ) after prefix() on a pure Sequence (Collections override this implementation) would loose the prefix, because in the internal drop(while: ) override grabs the underlying base iterator from _PrefixSequence and just wraps it in the _DropWhileSequence.

[palimondo]

I don’t have Swift 3.1 to test this myself, but I think this would be the test case:

let xs = sequence(first: 1, next: {$0 < 10 ? $0 + 1 : nil}) // Equivalent to 1...10
let ys = xs.prefix(3).drop(while: {$0 < 7})

Array(ys) // should be [], but I guess it’s [7, 8, 9, 10]

Pinging CodaFi (JIRA User) who introduced this bug addressing comments by @gribozavr.

[CodaFi]

#8087

[CodaFi]

This tracks 3.1 #8141

[dabrahams]

Works for me:

(swift) let xs = sequence(first: 1, next: {$0 < 10 ? $0 + 1 : nil}) // Equivalent to 1...10
// xs : UnfoldSequence<Int, (Int?, Bool)> = Swift.UnfoldSequence<Swift.Int, (Swift.Optional<Swift.Int>, Swift.Bool)>(_state: (Optional(1), true), _next: (Function), _done: false)
(swift) let ys = xs.prefix(3).drop(while: {$0 < 7})
// ys : AnySequence<Int> = Swift.AnySequence<Swift.Int>(_box: Swift._SequenceBox<Swift.AnySequence<Swift.Int>>)
(swift)
(swift) Array(ys)
// r0 : Array<Int> = []
(swift)

[palimondo]

@dabrahams, looks like you've tested this on master after 81968e2 already landed… Should I reopen this? Or did you mean to resolve it as fixed instead of can't reproduce?

[dabrahams]

You're quite right; this should be resolved as fixed. I see that shipping Swift gets this wrong. I don't really understand why, but I suppose it doesn't matter.

[palimondo]

@dabrahams What went wrong, process-wise, that this shipped in 3.1?! //@airspeedswift

[palimondo]

The fix was integrated into Swift 3.1 branch and is waiting for a release in version 3.1.1. This hasn't shipped yet as of Xcode 8.3.2.

[palimondo]

I stand corrected. The fix is in Xcode 8.3.2. Sorry for the confusion. Closing.