[SR-1043] Cannot Convert Array of Types Conforming to a Protocol to an Array of that Protocol Type

[swift-ci]

Previous ID	SR-1043
Radar	None
Original Reporter	Ben-G (JIRA User)
Type	Bug
Status	Reopened
Resolution

Environment

Xcode 7.3, Swift 2.2

Additional Detail from JIRA

Votes	11
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 5933d8df40f388055dd4c8c47d23ca7e

relates to:

• SR-352 Crash when implicitly converting array of [T] to a protocols [SomeProtocol]

Issue Description:

Swift currently cannot convert an array of types conforming to a protocol X to an array of [X]. It is possible to circumvent this by using `map`.

Example:

protocol AnotherThing {}

struct X: AnotherThing {}

let a = [X(), X(), X()]

func takesX(a: [X]) -> [AnotherThing] {
    // Compilation of following line fails
    let anotherThings: [AnotherThing] = a as [AnotherThing]
    //  This works:
    //  let anotherThings: [AnotherThing] = a.map { $0 as AnotherThing }

    return anotherThings
}

func takesOneX(a: X) -> AnotherThing {
    return a
}

The example project is also available on GitHub: https://github.com/Ben-G/AppleBugProjects/blob/master/ProtocolArrayTypeConversion/ProtocolArrayTypeConversion/main.swift