Generics with Protocols - Separate files

Using Swift
#1

Hi,

When using Generics with Protocols I get a compilation error depending on whether or not conformance to a protocol is defined in the same file or not.

Error:
Type 'Car.ItemID' (aka 'CarID') does not conform to protocol 'P2'

I have 2 swift files:
- CarID.swift
- CreateCars.swift

Observations:
- I get the error when extension CarID : P2 {} is defined in CarID.swift
- I don’t get the error when extension CarID : P2 {} is defined in CreateCars.swift as shown in the comments below.

Tested On:
- Xcode 9.0 (9A235) - GM
- Command Line project / iOS project

CarID.swift:

import Foundation

protocol P1 {
    associatedtype ItemID
}

class Car : P1 {
    typealias ItemID = CarID
}

//extension CarID : P2 {} //Error wouldn't occur if this line is uncommented and the line in CarID.swift is commented out

protocol P2 {}

class CreateItems<Something : P1>
where Something.ItemID : P2 {}

class CreateCars : CreateItems<Car> {} //Error: Type 'Car.ItemID' (aka 'CarID') does not conform to protocol 'P2'

extension CreateCars {}

CreateCars.swift:

import Foundation

class CarID {}

extension CarID : P2 {} //If this line is moved to CreateCars.swift the error vanishes away

Thanks and regards,
Muthu

#2

Sorry about the typo (filenames for the code were mentioned wrong).

The corrected filenames are as follows (problem still occurs):

Question:
- Why does the error occur when extension CarID : P2 {} is defined in CarID.swift ?

CreateCars.swift:

import Foundation

protocol P1 {
    associatedtype ItemID
}

class Car : P1 {
    typealias ItemID = CarID
}

//extension CarID : P2 {} //Error wouldn't occur if this line is uncommented and the line in CarID.swift is commented out

protocol P2 {}

class CreateItems<Something : P1>
where Something.ItemID : P2 {}

class CreateCars : CreateItems<Car> {} //Error: Type 'Car.ItemID' (aka 'CarID') does not conform to protocol 'P2'

extension CreateCars {}

CarID.swift:

import Foundation

class CarID {}

extension CarID : P2 {} //If this line is moved to CreateCars.swift the error vanishes away

Thanks and regards,
Muthu

···

On 17 Sep 2017, at 10:55 AM, somu subscribe via swift-users <swift-users@swift.org> wrote:

Hi,

When using Generics with Protocols I get a compilation error depending on whether or not conformance to a protocol is defined in the same file or not.

Error:
Type 'Car.ItemID' (aka 'CarID') does not conform to protocol 'P2'

I have 2 swift files:
- CarID.swift
- CreateCars.swift

Observations:
- I get the error when extension CarID : P2 {} is defined in CarID.swift
- I don’t get the error when extension CarID : P2 {} is defined in CreateCars.swift as shown in the comments below.

Tested On:
- Xcode 9.0 (9A235) - GM
- Command Line project / iOS project

CarID.swift:

import Foundation

protocol P1 {
    associatedtype ItemID
}

class Car : P1 {
    typealias ItemID = CarID
}

//extension CarID : P2 {} //Error wouldn't occur if this line is uncommented and the line in CarID.swift is commented out

protocol P2 {}

class CreateItems<Something : P1>
where Something.ItemID : P2 {}

class CreateCars : CreateItems<Car> {} //Error: Type 'Car.ItemID' (aka 'CarID') does not conform to protocol 'P2'

extension CreateCars {}

CreateCars.swift:

import Foundation

class CarID {}

extension CarID : P2 {} //If this line is moved to CreateCars.swift the error vanishes away

Thanks and regards,
Muthu
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