[SR-3038] Cannot pass in a protocol type to a constrained generic parameter

[swift-ci]

Previous ID	SR-3038
Radar	None
Original Reporter	plivesey (JIRA User)
Type	Bug
Status	Resolved
Resolution	Duplicate

Environment

Swift 3, playground

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 2fb3ab1d24786f3528652546206fc0aa

duplicates:

• SR-55 non-@objc protocol existentials do not conform to their own protocol type

Issue Description:

If a generic specifies <T: SomeProtocol>, you cannot pass in an object with type SomeProtocol. This doesn't make sense since presumably, a type should conform to itself.

To reproduce, put this in a playground:

protocol P {}

class A: P {}

func foo<T: P>(_ arg: T) {
}

let a = A()
foo(a) // compiles

foo(a as P) // fails to compile

Expected: Compilation succeeds
Actual: Compilation failed

[belkadan]

This is correct in the general case: not all protocols conform to themselves. The simplest case is a protocol with initializer requirements: which concrete type would you construct?

But for protocols that do allow this, is it okay to use them in this way? It's tricky.

[swift-ci]

Comment by Peter Livesey (JIRA)

I did wonder if there was a edge case like that that I hadn't forseen. That does make sense.

Presumably, Swift doesn't currently split between these types of protocols (reasonable). In that case, is it possible to have another error message for this? Like: "Protocol types cannot be used as a variable bound to a generic constraint." Or something like that which is worded better and more correct terminology?