AsyncSequence subprotocol with primary associated type, erase with existential, and check try-await element type

AsyncSequence-subprotocol-try-await-element-type.swift

import Foundation
import _Concurrency

extension AsyncStream {
    public init<S: AsyncSequence & Sendable>(
        _ sequence: S,
        bufferingPolicy limit: Continuation.BufferingPolicy = .unbounded
    ) where S.Element == Element {
        self.init(bufferingPolicy: limit) { continuation in
            let task = Task {
                do {
                    for try await element in sequence {
                        continuation.yield(element)
                    }
                } catch {}
                continuation.finish()
            }

            let terminationClosure: @Sendable (Continuation.Termination) -> Void = { _ in
                task.cancel()
            }

            continuation.onTermination = terminationClosure
        }
    }
}

// MARK: ----------------------------------------

// Using Swift 5.7 primary associated type
protocol MyAsyncSequence<Element>: AsyncSequence {}

extension AsyncStream: MyAsyncSequence {}

let stream = AsyncStream<Int> { nil }

let anyMySequence: any MyAsyncSequence<Int> = stream

// ERROR: x is Any, not Int
for try await x: Int in anyMySequence {

}

let mySeqStream = AsyncStream.init(anyMySequence) // NOTE: This `init` is type-erasure

// OK: x is Int
for try await x: Int in mySeqStream {

}



"✅"