[SR-2792] Bad diagnostic attempting to use typealias in protocol

[ddunbar]

Previous ID	SR-2792
Radar	None
Original Reporter	@ddunbar
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, DiagnosticsQoI
Assignee	@slavapestov
Priority	Medium

md5: 973fc691a7ee13b82cbb06954e89cfda

Issue Description:

This diagnostic is unfortunate:

$ cat z.swift 
protocol P {
    typealias Thing = Int
}

struct S: P {
    let x = Thing(1)
}

$ swiftc -c z.swift 
z.swift:6:13: error: use of unresolved identifier 'Thing'
    let x = Thing(1)
            ^~~~~
z.swift:2:15: note: did you mean 'Thing'?
    typealias Thing = Int
              ^

... I did Swift, I did.

[belkadan]

@DougGregor, who was working on this?

[DougGregor]

@slavapestov was looking at this at one point, and it sorta ended up on me with name lookup.

[ddunbar]

For my own sanity, can someone explain what is actually happening here a little bit? I've always been kinda confused on what a typealias assignment inside a protocol actually means (and can do).

[belkadan]

It used to be an associated type default, but now that that has its own keyword it should just be treated as a normal typealias, like one found in a struct or class. The tricky bit was if the typealias referenced Self or any associated types, though, and all the existing code that avoided looking up types in protocols is still there.

[ddunbar]

Ah ha, ok that is what I suspected might be going on. Thanks!

[slavapestov]

Typealiases in protocols mostly work, but unqualified lookup cannot find them.

[slavapestov]

#6552