That seems to be a known problem: https://bugs.swift.org/browse/SR-773
<https://bugs.swift.org/browse/SR-773>
> On 29. Apr 2017, at 22:38, Nevin Brackett-Rozinsky via swift-users
> <swift-users@swift.org> wrote:
>
> 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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