[SR-6918] `ambiguous reference to member` error with conditional conformance and subprotocol

[rdingman]

Previous ID	SR-6918
Radar	rdar://problem/39193393
Original Reporter	@rdingman
Type	Bug
Status	Resolved
Resolution	Done

Attachment: Download

Environment

Xcode 9.3 beta

Additional Detail from JIRA

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

md5: 12f73c06261b39fb3e0284b3416f78de

Issue Description:

These seems like it should work, but please correct me if I am wrong. I'm getting a `ambiguous reference to member` error with a conditional conformance to a protocol P and using a subprotocol inheriting from protocol P when protocol P has a default implementation via a protocol extension.

The attached playground demonstrates what I'm running into.

[belkadan]

You stated Wrapped == ProtocolOne rather than Wrapped: ProtocolOne, but both have problems. The former works on Optionals whose wrapped value is exactly a protocol value of that type, so optionals with the wrapped type ProtocolTwo don't see it (even though you can convert from one to the other). The latter works for optionals wrapping concrete types, but not for arbitrary protocol values, since you might try to do something like instantiate a new value of that type. I think you might be stuck.

@DougGregor, anything to add?

[huonw]

This looks like a diagnostics issue: as Jordan says, the Wrapped == ProtocolOne means the conformance only applies to exactly ProtocolOne?, not ProtocolTwo?. I think the error message should be making it clearer that neither of the functions actually work at all, because the conformance to ProtocolOne doesn't apply.

[huonw]

@swift-ci create

[xedin]

Looks like starting from 5.2 we produce a much better diagnostic here:

error: property 'property' requires the types 'ProtocolTwo' and 'ProtocolOne' be equivalent
print(value2.property)
             ^
note: where 'Wrapped' = 'ProtocolTwo'
extension Optional: ProtocolOne where Wrapped == ProtocolOne {
^

@rdingman Please use the latest master/5.2 snapshot to verify and close.