[SR-1976] Closure signature in Swift 3 required for inout params

[swift-ci]

Previous ID	SR-1976
Radar	None
Original Reporter	erica (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Attachment: Download

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug
Assignee	erica (JIRA)
Priority	Medium

md5: 5e016df9563a1b6894b80711441939f1

is duplicated by:

• SR-3632 Swift is unable to properly infer the type of closures with inout parameters without specific type information.

relates to:

• SR-3520 Generic function taking closure with inout parameter can result in a variety of compiler errors or EXC_BAD_ACCESS

Issue Description:

For whatever reason, you have to declare the closure signature with inout variables in Swift 3. You didn't in Swift 2. Example below. Also affected the new stdlib sequence functions.

Swift 2:

func myFor<T>(args: T, _ test: (T) -> Bool, _ next: (inout T) -> Void, body: (inout T) -> Void) {
    var state = args
    while test(state) {
        defer { next(&state) } ; body(&state)
    }
}

myFor(0, { $0 < 10 }, { $0 += 2 }) {
    print($0)
}

Swift 3:

func myFor<T>(_ args: T, _ test: (T) -> Bool, _ next: (inout T) -> Void, body: (inout T) -> Void) {
    var state = args
    while test(state) {
        defer { next(&state) } ; body(&state)
    }
}

myFor(0, { $0 < 10 }, { (i: inout Int) in  return i += 2 }) {
    print($0)
}

[belkadan]

Hm. I don't remember this ever working, even in Swift 2. (I believe you that it compiled, but I also remember getting errors sometimes, which makes me think it may not have been implemented correctly.) @slavapestov, do you know what changed here?

[swift-ci]

Comment by erica sadun (JIRA)

Just for the record:

Also, I'd really like this not to be a requirement: is there a solid language reason for making it hard to use closures with inout?

[swift-ci]

Comment by erica sadun (JIRA)

Changing code to the following works in Swift 3/Xcode 8 (thanks Mike Ash):

func myFor<T>(_ args: T, _ test: (T) -> Bool, _ next: (inout T) -> Void, body: (inout T) -> Void) {
    var state = args
    while test(state) {
        defer { next(&state) } ; body(&state)
    }
}

myFor(0, { $0 < 10 }, { $0 = $0 + 2 }) {
    print($0)
}

So it may be an issue with +=

[swift-ci]

Comment by erica sadun (JIRA)

Further: Mike Ash offers this, which crashes too:

func myFor<T>(_ args: T, _ test: (T) -> Bool, _ next: (inout T) -> Void, body: (inout T) -> Void) {
    var state = args
    while test(state) {
        defer { next(&state) } ; body(&state)
    }
}

let plusEquals: (inout Int, Int) -> Void = (+=)
myFor(0, { $0 < 10 }, { plusEquals(&$0, 2) }) {
    print($0)
}

[rudkx]

Fixed on master with this merge: c04c6c9

Please confirm with a build that has this fix in it and close the issue if you agree the issue is fixed.