[swift-users] Swift 4 "Cannot use mutating member on immutable value: 'self' is immutable"
Quinn "The Eskimo!"
eskimo1 at apple.com
Thu Sep 14 03:13:58 CDT 2017
On 14 Sep 2017, at 03:56, somu subscribe via swift-users <swift-users at swift.org> wrote:
> popFirst is not available in the Array …
Right. This makes sense when you consider the standard setup for an array, namely, a variable length buffer of items. Removing the first element is expensive, whereas removing the last element (`popLast()`) is cheap.
If you run your simplified example in Xcode 8 you get an error that hints at what’s going on.
var array = [1, 2, 3, 4, 5]
_ = array.popFirst()
error: 'ArraySlice<Int>' is not convertible to '[Int]'
Notably, I put Rick’s code into Xcode 8 (Xcode 8.3.3 on macOS 10.12.6 with a new command line tool project) and I get the same error there.
let mf = self.pendingFetchers.popFirst()
error: 'ArraySlice<ModelFetcher>' is not convertible to '[ModelFetcher]’
* * *
@Rick, there’s two things in play here:
* The diagnostic is clearly bogus and you should definitely file a bug about that.
Please post your bug number, just for the record.
* You wrote:
> This code compiled fine in Xcode 8 …
My tests indicate that it doesn’t. I suspect that you changed something else during the Swift 4 migration and that’s why you’re seeing it fail. Can you take another look at the original Xcode 8 code to see what’s else got changed?
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Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
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