Understanding SE-0346: Can P<T> be Used for Protocol Conformance?

ensan-hcl · June 18, 2023, 1:22am

In SE-0346, it is said that

In the inheritance clause of a concrete type, for example:
struct Lines : Collection<String> { ... }
In this position it is sugar for specifying the associated type witness, similar to explicitly declaring a typealias:
struct Lines : Collection {
  typealias Element = String
}

It seems that in 'three other positions' the proposal mentions, only this usage cannot be compiled in Swift 5.8.

protocol P<T> {
    associatedtype T
}
// Cannot inherit from protocol type with generic argument 'P<Int>'
struct F: P<Int> {
    typealias T = Int
}

I'd like to clarify that whether this is due to a bug, or this is just not implemented, or this is an expected behavior. Does anyone know what is happening here?

ensan-hcl · June 18, 2023, 2:49pm

Reported as an issue

github.com/apple/swift

[Swift 5.8] Unexpected error "Cannot inherit from protocol type with generic argument"

opened 02:49PM - 18 Jun 23 UTC

ensan-hcl

bug triage needed

**Description** In Swift 5.8, when attempting to inherit from a protocol with p…rimary associated types specified and providing a typealias for the associated type, the code fails to compile with the error "Cannot inherit from protocol type with generic argument 'P<Int>'." **Steps to reproduce** 1. Define a protocol `P` with a primary associated type `T`. 2. Attempt to create a struct `F` that inherits from `P<Int>` and provide a typealias for the associated type. ```swift protocol P<T> { associatedtype T } // Cannot inherit from protocol type with generic argument 'P<Int>' struct F: P<Int> {} ``` **Expected behavior** The code should compile without any errors, as mentioned in [SE-0346](https://github.com/apple/swift-evolution/blob/main/proposals/0346-light-weight-same-type-syntax.md#other-positions) > In the inheritance clause of a concrete type, for example: > ```swift > struct Lines : Collection<String> { ... } > ``` > In this position it is sugar for specifying the associated type witness, similar to explicitly declaring a typealias: > ```swift > struct Lines : Collection { > typealias Element = String >} > ``` **Environment** - Swift compiler version info: swift-driver version: 1.75.2 Apple Swift version 5.8.1 (swiftlang-5.8.0.124.5 clang-1403.0.22.11.100) Target: x86_64-apple-macosx13.0 - Xcode version info: Xcode 14.3.1 Build version 14E300c - Deployment target: N/A **Other Information** * [Post in Swift Forums](https://forums.swift.org/t/understanding-se-0346-can-p-t-be-used-for-protocol-conformance/65641)