[SR-2948] Discarding a closure from a @discardableResult function results in a compile error.

[swift-ci]

Previous ID	SR-2948
Radar	None
Original Reporter	Amnykon (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, StarterBug
Assignee	None
Priority	Medium

md5: 021904a7cf17206d058f15ce5ed4b66f

relates to:

• SR-7297 Discarding a function from a discardableResult method causes a compiler error

Issue Description:

A function marked with @discardableResult should not throw an error if it is returning closure is unused.

The current work around is to set _ to the returning closure.

@discardableResult func run (_ closure: @escaping ()->()) -> (()->()) {
  closure()
  return closure
}

var closure={print(1)}
_ = run(closure)
// Prints 1

_ = run(run(closure))
// Prints 1
// Prints 1

run(closure)
// should Print 1 but errors with:
//   discardableResultOfClosure.swift:14:1: error: expression resolves to an unused function
//   run(closure)
//   ^~~~~~~~~~~~

[swift-ci]

Comment by Erik Verbruggen (JIRA)

Matthew: Oh, sorry, yes: #6785

Please do comment on that patch! 🙂

[swift-ci]

Comment by Erik Verbruggen (JIRA)

@belkadan should I also close this issue now that the pull-request has merged?

[belkadan]

Usually "resolved" means the implementer thinks it's done and "closed" means the reporter thinks it's done. So we can leave it here for Micah to verify or not.

[hamishknight]

Found a variant of this bug with methods: https://bugs.swift.org/browse/SR-7297

[swift-ci]

Comment by Rafael Gutierrez (JIRA)

I'm still seeing this issue happen for swift 4.2.

[belkadan]

You sure you're not seeing SR-7297?

[swift-ci]

Comment by Rafael Gutierrez (JIRA)

In reality @belkadan it might be a different issue altogether. I have the following code:

@discardableResult func testFunc() -> () -> Void {
    return {
        print("yehaw")
    }
}

//call function
testFunc() // Expression resolves to an unused function

Let me know if you think this issue is unrelated to this one and I can file a separate issue. It does not seem related to SR-7297 .

[belkadan]

That's not producing any message for me when using the Swift that comes with Xcode 10.0 (from the command line). How are you compiling this?