[SR-453] Structs, Protocols and casting

[swift-ci]

Previous ID	SR-453
Radar	None
Original Reporter	flashspys (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, LanguageFeatureRequest
Assignee	None
Priority	Medium

md5: 973efe727beed10b6f2ae35767f6d7b6

Issue Description:

I don't like the way how Swift handles structs which implements protocols:

Simple demonstration:

protocol Prot {}
struct A : Prot {}

let arr = [A(), A()] // 1: ok
let arr2 = [A(), A()] as [Prot] // 2: ok
let arr3 = arr as [Prot] // 3: compile error
let arr4 = arr2 as! [A] // 4: runtime error

It was discussed here. I think it's mandatory to support casting here and should not be a language limitation. I really like structs in swift but it is very hard to implement sth. like a hierarchy of structs cause they can't inherit from each other and it's hard to work with protocols as you see above.

Does anyone feels similar ?

[Dante-Broggi]

I think this can be closed, neither error occurs in the Xcode 9.4.1 toolchain.

[AnthonyLatsis]

We got test coverage for this here—expect for the arr4 case, which expectedly works, but I'm going to add it for the record anyway before closing.