[swift-dev] Foundation Data Behavior Different (read: crashes) in Swift 3.1 and Swift 3.2

[Philippe Hausler]

I see the issue.. the latest version traps (appropriately so).

let str = String(bytes: data[0..<2], encoding: .utf8)!

The sub-range of the slice you have is incorrectly indexed. Try this out (I am presuming this is what you mean):

let str = String(bytes: data[data.startIndex..<(data.startIndex + 2)], encoding: .utf8)!

> On Aug 24, 2017, at 11:07 AM, Philippe Hausler <phausler at apple.com> wrote:
> 
> Is there a radar or bugs.swift.org ticket filed on this?
> 
> I presume because of the import this is a Darwin thing and not a linux thing.
> 
>> On Aug 24, 2017, at 11:05 AM, Michael Gottesman via swift-dev <swift-dev at swift.org> wrote:
>> 
>> 
>>> On Aug 24, 2017, at 10:47 AM, Ryan Lovelett via swift-dev <swift-dev at swift.org> wrote:
>>> 
>>> I've found what I believe is a bug. Though I'm unclear if the bug is in
>>> Swift 3.1 or Swift 3.2/4.0. All I can say for sure is the behavior is
>>> quite drastically different between the two.
>>> 
>>> For the code below (and attached):
>>> 
>>>  import Cocoa
>>> 
>>>  var data = Data(bytes: [0x50, 0x4B, 0x01, 0x02, 0x41, 0x61])
>>>  data.removeFirst(4)
>>>  let base64 = data.base64EncodedString()
>>>  let str = String(bytes: data[0..<2], encoding: .utf8)!
>>>  print("Base64: \(base64), String: \(str)")
>>> 
>>> If I compile and run that with the Swift included in Xcode 8.3.3 (e.g.,
>>> swift ./data-bug.swift) it outputs: Base64: QWE=, String: Aa. Which is
>>> what I expect.
>>> 
>>> With the Swift that is included with Xcode 9.0 beta 6 (9M214v) (e.g.,
>>> swift -swift-version 3 ./data-bug.swift). It performs an illegal
>>> hardware instruction and crashes. It also does this if I use use the
>>> version 4 of the compiler.
>>> 
>>> Is this a bug? If so where is the bug? Was this always meant to not work
>>> and Swift 3.1 just happened to work or is there now an issue in the
>>> Swift 3.2 implementation?
>> 
>> I have not engaged my brain with the particulars of the rest of the email, but high level question: does this happen without optimization? Or does it happen only with optimization?
>> 
>> Michael
>> 
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