[swift-users] Can't implement generic subscript in protocol

[Slava Pestov]

> On Sep 15, 2017, at 4:26 PM, Slava Pestov <spestov at apple.com> wrote:
> 
> 
>> On Sep 15, 2017, at 12:40 PM, Joanna Carter via swift-users <swift-users at swift.org> wrote:
>> 
>> Xcode Version 9.0 (9A235)
>> 
>> Hmmm. I'm now getting a segmentation fault : 11 on a constructor when trying to implement type erasure :
> 
> Swift 4.0 had issues with members and member references named ‘subscript’. On master, I get the correct error message without crashing:
> 
> Slavas-MBP:swift slava$ ../build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swiftc sub.swift 
> sub.swift:22:17: error: value of type 'providerType' has no member 'subscript'
>   _subscript = base.subscript
>                ^~~~ ~~~~~~~~~
> 
> Indeed you’re not supposed to be able to refer to a subscript member like this. You could use a keypath, or try some other approach.

You could also write an explicit closure that loads the subscript:

let fn = { base[$0] }

Slava

> 
> Slava
> 
>> 
>> protocol DataProvider
>> {
>> associatedtype ItemType
>> 
>> subscript(index: Int) -> ItemType { get }
>> }
>> 
>> class _AnyDataProviderBoxBase<itemType>: DataProvider
>> {
>> subscript(index: Int) -> itemType
>> {
>>   fatalError()
>> }
>> }
>> 
>> class _AnyDataProviderBox<providerType: DataProvider>: _AnyDataProviderBoxBase<providerType.ItemType>
>> {
>> private let _subscript: (_ index: Int) -> providerType.ItemType
>> 
>> init(_ base: providerType) // segmentation fault : 11
>> {
>>   _subscript = base.subscript
>> }
>> 
>> override subscript(index: Int) -> providerType.ItemType
>> {
>>   return _subscript(index)
>> }
>> }
>> 
>> final class AnyDataProvider<providerType : DataProvider>: DataProvider
>> {
>> private let box: _AnyDataProviderBox<providerType>
>> 
>> init(_ base: providerType)
>> {
>>   self.box = _AnyDataProviderBox(base)
>> }
>> 
>> subscript(index: Int) -> providerType.ItemType
>> {
>>   return box[index]
>> }
>> }
>> 
>> Joanna
>> 
>> --
>> Joanna Carter
>> Carter Consulting
>> 
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>