[SR-5719] Ban autoclosure forwarding

[DevAndArtist]

Previous ID	SR-5719
Radar	rdar://problem/37321597
Original Reporter	@DevAndArtist
Type	Bug
Status	Resolved
Resolution	Done

Environment

Xcode 8.3.3

Additional Detail from JIRA

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

md5: 401a3fa1c8f5f08e5385cee515f04b79

is duplicated by:

• SR-9494 Disable @autoclosure forwarding and provide a fix-it to add ()

Issue Description:

func autoclosureFoo(_ c: @autoclosure () -> Int) {
    print(c())
}

func autoclosureBar(_ c: @autoclosure () -> Int) {
    autoclosureFoo(c) // <-- This one here
    autoclosureFoo(c())
}

func baz(_ c: () -> Int) {
    autoclosureFoo(c) // does not work as expected
}

autoclosureBar(42)

If this requires a full review on the evolution side, let me know, I'll start a new thread.

[CodaFi]

Oh dear. This seems to have been the result of a bad conditional left over when fixing SR-832.

[CodaFi]

To expand: The rules for matching parallel @autoclosure function types are

• If doing a conversion, strip off the Void input from the second type and recur matching the result against the first type.

• If doing equality, try parallel matches. Both types must have @autoclosure to do so.

The condition in SR-832 instead checks for a mismatch and uses that as evidence it is doing a conversion which, being the identity, succeeds trivially. I'm not sure we can cut this off without severely breaking source (naive fix I have for this indicates that XCTest and Foundation are relying on this).

[DevAndArtist]

Yet it's an abuse of a bug, similar like some people abuse

guard let `self` = self else { ... }

in closures with `weak` self.

I see two options (there is third one, but I won't be happy with it in a long term):

1. Fix this issue (provide migration for this `add () at the ende of the forwarded autoclosure`)

2. Align autoclosure with normal closures to accept normal explicit closures with the same signature (however this might have breaking changes in generic area - if I'm wrong here, then this change is more than welcome)

func foo() -> Int { return 42 }
func bar(_ c: @autoclosure () -> Int) { ... }
bar(foo) 

This would allow us to specify autoclosures with default reference to another function/closure:

func foo() -> Int { return 42 }
func bar(_ c: @autoclosure () -> Int = foo) { ... }
bar() 

3. Live with this asymmetric bug.

[belkadan]

I think it does need to go to swift-evolution, since it will break existing code. A core team member could rule that it doesn't need a full review, but it should at least come up there.

[xedin]

@swift-ci create

[xedin]

@autoclosure forwarding has been banned starting from `swift-version 5`.