[swift-evolution] Removing "_ in" from empty closures

[John McCall]

> On May 19, 2016, at 4:13 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org> wrote:
>> On May 14, 2016, at 22:16, Chris Lattner via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> On May 13, 2016, at 9:16 AM, Joe Groff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> This encourages the use of empty closures over optional closures, which I think is open for debate. In general I try to avoid optionals when they can be precisely replaced with a non-optional value. Furthermore, most Cocoa completion handlers are not optional.
>>>> 
>>>> The alternative is to not do this, but encourage that any closure that could reasonably be empty should in fact be optional. I would then want Cocoa functions with void-returning closures to be imported as optionals to avoid "{ _ in }".
>>> 
>>> +1. In general, I think we should allow implicit arguments, without requiring the closure to use all the implicit $n variables like we do today. These should all be valid:
>>> 
>>> let _: () -> () = {}
>>> let _: (Int) -> () = {}
>>> let _: (Int, Int) -> Int = { 5 }
>>> let _: (Int, Int) -> Int = { $0 }
>>> let _: (Int, Int) -> Int = { $1 }
>> 
>> I agree, but I consider this to be an obvious bug in the compiler.  I don’t think it requires a proposal.
> 
> Sorry to find this thread late. I don’t think this is just a bug; it’s also a way to check that a parameter isn’t getting forgotten. For a single-expression closure that’s probably overkill, but maybe we’d keep the restriction for multi-statement closures?

The bug we're talking about is that closures have to have a reference to $n when there are n+1 parameters.

John.
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