[SR-2324] Optional closure type is always considered @escaping

[swift-ci]

Previous ID	SR-2324
Radar	rdar://problem/27795901
Original Reporter	ilijatovilo (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

OSX 10.11.6 (15G31)
MacBook Pro (Retina, Mid 2012)
Version 8.0 beta 5 (8S193k)
Toolchain: Swift Development Snapshot 2016-08-07 (a)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	@milseman
Priority	Medium

md5: ab4ee203bb7d77d3976562cd10a4342c

is duplicated by:

• SR-2166 Should be able to @NoEscape a optional closure
• SR-2444 @escaping failing on optional blocks
• SR-2474 closure in protocol extension implicitly captures mutating 'self'

Issue Description:

The following code snippet doesn't work anymore in the current Swift 3 snapshot:

func takes(anOptionalClosure: (() -> Void)?) {
    if let aClosure = anOptionalClosure {
        aClosure()
    }
}

func takes(aClosure: () -> Void) {
    takes(anOptionalClosure: aClosure)
}

> main.swift:18:30: Cannot convert value of type '() -> Void' to expected argument type '(() -> Void)?'

My guess is that the type checker thinks that `(() -> Void)?` is interpreted as a tuple type. But the parens are only necessary to make the closure optional. Interestingly this only happens with function types. `(String)?` e.g. type checks correctly.

The following snippet works as expected:

func takes(anOptionalClosure: (() -> Void)?) {
    if let aClosure = anOptionalClosure {
        aClosure()
    }
}

// Hint: Added two surrounding parens to the argument type
func takes(aClosure: (() -> Void)) {
    takes(anOptionalClosure: aClosure)
}

[belkadan]

The actual issue here is that optional closures are implicitly @escaping right now.

[belkadan]

@milseman, do we have a JIRA or Radar for this already?

[milseman]

Yes, <rdar://problem/27795901>

[swift-ci]

Comment by Shawn Erickson (JIRA)

Note I added a comment to SR-2444 which is marked resolved because I assume it is considered a duplicate of this issue. What is described in SR-2444 is problematic still in Xcode 8 GM.

https://bugs.swift.org/browse/SR-2444?focusedCommentId=17861&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-17861

[milseman]

Also, rdar://problem/28372774

[milseman]

With #4905 produces the correct error message here:

...: error: unexpected note produced: parameter 'aClosure' is implicitly non-escaping
func takes(aClosure: () -> Void) {
           ^
...: error: unexpected error produced: passing non-escaping parameter 'aClosure' to function expecting an @escaping closure
    takes(anOptionalClosure: aClosure)
                             ^