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[SR-8753] wrong error message is given when you attempt to mutate parameters inside a reduce block #51261

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swift-ci opened this issue Sep 14, 2018 · 3 comments
Closed

[SR-8753] wrong error message is given when you attempt to mutate parameters inside a reduce block #51261

swift-ci opened this issue Sep 14, 2018 · 3 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation type checker Area → compiler: Semantic analysis

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@swift-ci
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Previous ID SR-8753
Radar rdar://problem/44476380
Original Reporter swiftHoney (JIRA User)
Type Bug
Status Resolved
Resolution Done
Environment

Swift 4.1.2

Xcode 9.4.1

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, DiagnosticsQoI, TypeChecker
Assignee swiftHoney (JIRA)
Priority Medium

md5: 2e445a28e7ba51855d170331b7ee7e6a

Issue Description:

As recorded on stackoverflow:

{code:none}
var nums = [1,2,3]
let emptyArray : [Int] = []
let sum1 = nums.reduce(emptyArray){ $0.append($1)}

would result in the following error:

{quote}Type of expression ambiguous without more context
{quote}

But that error is incorrect. 

If I wrote as: 

{code:none}
func append<T>(_ element: T, to array: [T]) -> [T] {
  let newArray = array.append(element)
  return newArray
}

Then the error would correctly come as:

Cannot use mutating member on immutable value: 'array' is a 'let' constant
@belkadan
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Reproduces on master. cc @xedin

@xedin
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xedin commented Sep 15, 2018

@swift-ci create

@hborla
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hborla commented Feb 6, 2020

This is resolved in Swift 5.2. The compiler now reports two error messages:

error: cannot convert value of type '()' to closure result type '[Int]'
let sum1 = nums.reduce(emptyArray){ $0.append($1) }
                                    ~~~^~~~~~~~~~
error: cannot use mutating member on immutable value: '$0' is immutable
let sum1 = nums.reduce(emptyArray){ $0.append($1) }
                                    ~~ ^

Could you please verify using a recent 5.2 snapshot, or the latest Xcode beta? Thank you!

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation type checker Area → compiler: Semantic analysis
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@xedin @belkadan @hborla @swift-ci