[swift-users] Error creating empty array of chained associated type

[Nevin Brackett-Rozinsky]

I’ve stumbled upon an odd situation where Swift gives a compiler error if I
do things directly, but works properly with no error if I first create a
typealias. Here is a stripped-down example:

protocol P {
    associatedtype C: Collection
}

extension P {
    func emptyArray() -> [C.Iterator.Element] {
        return [C.Iterator.Element]()   // Error
    }
}

The “return” line gives the error “Cannot call value of non-function type
'[Self.C.Iterator.Element.Type]'” in Xcode 8.3.2. However, if we replace
that function with a seemingly-equivalent version that uses a typealias,
there is no error:

extension P {
    func emptyArray() -> [C.Iterator.Element] {
        typealias E = C.Iterator.Element
        return [E]()    // Works
    }
}

Is this a known bug?

Nevin
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