[SR-5869] trailing closure not parsed correctly in if-statement conditional #48439

swift-ci · 2017-09-11T19:24:58Z


Previous ID	SR-5869
Radar	https://bugreport.apple.com/web/?problemID=31489013
Original Reporter	tncowart (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

Xcode 8.3, Xcode 9 beta 6, macOS 10.12.6

Additional Detail from JIRA


Votes	0
Component/s	Compiler
Labels	Bug, Parser
Assignee	owenvoorhees (JIRA)
Priority	Medium

md5: ad36f443386a825f7e352edd3209ab5c

is duplicated by:

SR-10509 Optional map with nil coalesce fails to compile in if-statement
SR-11031 bad diagnostics with trailing closure in condition expression
SR-8736 Bad diagnostic for use of trailing closures in an if

Issue Description:

Summary: When using a function that takes a trailing closure as part of an if-statement conditional, writing the closure in the idiomatic `function { %0.thing }` style causes a compiler error. Writing the closure in the non-idiomatic `function({ $0.thing })` style works, though.

Steps to Reproduce:

Add the following to a file (`closure.swift`):

let nums = [1, 3, 5, 9]
if nums.filter { $0 % 2 == 0 }.isEmpty { 
    print("contains no even numbers") 
}

compile with swift closure.swift

Expected Results: I expect the code to compile and "contains no even numbers" to be printed to the console

Actual Results: the compiler throws multiple errors due to being unable to parse the conditional correctly. Specifically:

swift_bug.swift:5:18: error: anonymous closure argument not contained in a closure
if nums.filter { $0 % 2 == 0 }.isEmpty {
                 ^
swift_bug.swift:5:31: error: consecutive statements on a line must be separated by ';'
if nums.filter { $0 % 2 == 0 }.isEmpty {
                              ^
                              ;
swift_bug.swift:5:9: error: '((Int) throws -> Bool) throws -> [Int]' is not convertible to 'Bool'
if nums.filter { $0 % 2 == 0 }.isEmpty {
   ~~~~~^~~~~~
swift_bug.swift:6:5: error: ambiguous reference to member 'print(_:separator:terminator:)'
    print("no even numbers")
    ^~~~~

Notes:
This conditional works as expected:

if nums.filter({ $0 % 2 == 0}).isEmpty { 
    print("contains no even numbers") 
}

This gives a slightly different compilation error as above (error: missing argument for parameter #1 in call):

if nums.filter() { $0 % 2 == 0}.isEmpty { 
    print("contains no even numbers") 
}

{noformat}
swift_bug.swift:5:20: error: anonymous closure argument not contained in a closure
if nums.filter() { $0 % 2 == 0 }.isEmpty {
                   ^
swift_bug.swift:5:33: error: consecutive statements on a line must be separated by ';'
if nums.filter() { $0 % 2 == 0 }.isEmpty {
                                ^
                                ;
swift_bug.swift:5:16: error: missing argument for parameter #&#8203;1 in call
if nums.filter() { $0 % 2 == 0 }.isEmpty {
               ^
               <#(Int) throws -> Bool#>
Swift._ArrayProtocol:2:17: note: 'filter' declared here
    public func filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element]
                ^
swift_bug.swift:6:5: error: ambiguous reference to member 'print(_:separator:terminator:)'
    print("no even numbers")
    ^~~~~
{noformat}

The text was updated successfully, but these errors were encountered:

belkadan · 2017-09-12T17:24:34Z

We don't allow trailing closures in if-statements (for this sort of reason), but this is pretty poor recovery. @rintaro, @nkcsgexi, do you remember if there are any other bugs about improving the recovery?

rintaro · 2017-09-13T00:12:14Z

I don't think we have other bugs about this problem.

We currently recover only if closure is the outer most expression:

error: MyPlayground.playground:5:11: error: trailing closure requires parentheses for disambiguation in this context
if foo(1) { $0 + 42 } {  }
        ~~^
        , handler:   )

error: trailing closure requires parentheses for disambiguation in this context
if bar { $0 + 42 }, baz() {  }
      ~^
      (           )

We should expand our recovery logic, say "if } is immediately followed by [ . or ( on the same line, it must be a trailing closure."

belkadan · 2017-09-13T00:37:29Z

That makes sense. We could even look for the absence of a space, since people conventionally don't put spaces in for any of those.

belkadan · 2017-09-13T00:37:40Z

(but just checking the same line is probably sufficient)

nicklockwood · 2018-10-15T17:01:53Z

I see this bug hasn't been updated in a while, but just FYI I've stumbled upon a case where trailing closures are actually allowed in an if statement, but probably shouldn't be:

func someFuncWithClosure(closure: (Int) -> Bool) -> Bool {
    return closure(4)
}

// this prints "true", with no warnings
if true || someFuncWithClosure { $0 == 4 } {
    print("true")
}

mattneub · 2019-01-20T20:39:01Z

Still getting very unhelpful diagnostics from the compiler on this. I just encountered it as follows:

if let stat = stats.filter{$0.dateFinished == date}.first {}

Result is error messages as follows:

Anonymous closure argument not contained in a closure
Consecutive statements on a line must be separated by ';'
Initializer for conditional binding must have Optional type, not '((Stat) throws -> Bool) throws -> [Stat]'

But none of these gets at the heart of the matter, which is merely that we need to surround `stats.filter{$0.dateFinished == date}` with parentheses.

rintaro · 2019-04-18T19:37:55Z

I had PR for this here #12457 .
Need to revisit.

swift-ci · 2019-11-22T17:13:31Z

Comment by Owen Voorhees (JIRA)

Resolved in #28362

If possible, please verify and close using the next available master snapshot.

swift-ci transferred this issue from apple/swift-issues Apr 25, 2022

This issue was closed.

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[SR-5869] trailing closure not parsed correctly in if-statement conditional #48439

[SR-5869] trailing closure not parsed correctly in if-statement conditional #48439

swift-ci commented Sep 11, 2017

belkadan commented Sep 12, 2017

rintaro mannequin commented Sep 13, 2017

belkadan commented Sep 13, 2017

belkadan commented Sep 13, 2017

nicklockwood commented Oct 15, 2018

mattneub commented Jan 20, 2019

rintaro mannequin commented Apr 18, 2019

swift-ci commented Nov 22, 2019

[SR-5869] trailing closure not parsed correctly in if-statement conditional #48439

[SR-5869] trailing closure not parsed correctly in if-statement conditional #48439

Comments

swift-ci commented Sep 11, 2017

belkadan commented Sep 12, 2017

rintaro mannequin commented Sep 13, 2017

belkadan commented Sep 13, 2017

belkadan commented Sep 13, 2017

nicklockwood commented Oct 15, 2018

mattneub commented Jan 20, 2019

rintaro mannequin commented Apr 18, 2019

swift-ci commented Nov 22, 2019