SE-0286: Forward Scan for Trailing Closures

Evolution Proposal Reviews
15

I'm concerned about the compile-time performance impact of attempting N possible positions, but I absolutely agree that the potential for silently changing the meaning of code is a serious concern with this proposal. However, I think we can produce a meaningful warning by looking at the the type-checked call, which doesn't carry the same compile-time cost and should still be effective.

I went ahead and implemented a warning like you suggest, but without fully chasing down all of the solutions. It has a slightly higher chance of false positives because it doesn't check for complete wellformedness, but has a much lower implementation cost and risk. For your executeQuery example, it produces the following:

se0284.swift:12:14: warning: since Swift 5.3, unlabeled trailing closure argument matches parameter 'onSuccess' rather than parameter 'onCompletion'
executeQuery { _ in log("Query: \(query)") }
             ^
se0284.swift:12:14: note: label the argument with 'onCompletion' to retain the pre-Swift 5.3 behavior
executeQuery { _ in log("Query: \(query)") }
             ^
            (onCompletion:                  )
se0284.swift:12:14: note: label the argument with 'onSuccess' to silence this warning for Swift 5.3 and newer
executeQuery { _ in log("Query: \(query)") }
             ^
            (onSuccess:                     )
se0284.swift:5:6: note: 'executeQuery(onSuccess:onError:onCompletion:)' contains defaulted closure parameters 'onSuccess' and 'onCompletion'
func executeQuery(
     ^

Doug

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