[SR-12236] Getting "Circular reference" after update to Swift 5.2

[swift-ci]

Previous ID	SR-12236
Radar	rdar://problem/59640025
Original Reporter	RomanPodymov (JIRA User)
Type	Bug

Environment

Xcode 11.4 beta, Swift 5.2.

Additional Detail from JIRA

Votes	3
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 98af4db69d3431110b9adb14b03d7384

Issue Description:

Consider the following code:

protocol P { }
class A {
    typealias B = P
}
extension A: A.B { }

It is compiling without errors in Swift 5.1.3/Xcode 11.3.1, but in Swift 5.2/Xcode 11.4 beta getting error "Circular reference". I did not find information about such behavior in the Swift 5.2 release notes.

[CodaFi]

This was always a circularity issue, we're just seeing this surfaced now because we're actually catching this in the request evaluator. The trigger for the cycle is we attempt to evaluate A's conformance to any protocol - in this case Codable. This causes us to validate its inheritance clause, which requires resolving the reference to A.B. So we try qualified lookup for B in A then try to walk up to A's ancestors after we find the typealias. In order to walk to the set of protocols A conforms to, we have to go resolve the reference to A.B, and around and around we go.

This just happens to do the right thing on 5.1.

[CodaFi]

@swift-ci create

[swift-ci]

Comment by Roman Podymov (JIRA)

Hello @CodaFi
If this behavior is expected, I can set environment to Xcode 11.3.1, Swift 5.1.3 and close the issue.

[CodaFi]

It's expected, but it's also not great that we've managed to break source. Let's leave this open for now.

[swift-ci]

Comment by Ted Bradley (JIRA)

When I observe this in Xcode, the compiler gets stuck in an infinite loop. It would be good to at least terminate the compilation process when this happens.

[CodaFi]

tedbadley (JIRA User) Please file a separate issue and include your project sources. The evaluator specifically breaks infinite recursion, so I think you're observing a separate problem.

[swift-ci]

Comment by André Silva (JIRA)

Any news about this?
i'm getting this problem in a project that compiles with Xcode 11.3.1...but in newer Xcode is just fails with some **<unknown>:0: error: circular reference in files that are simple and don't implement the pattern in the description.

for example, it fails in https://github.com/apple/swift-package-manager/blob/1444b46dbc13477f027959e4d59e520747fc8382/swift-tools-support-core/Sources/TSCBasic/OrderedSet.swift

some of the other files are just simple plain struct models

[CodaFi]

afms (JIRA User) Circularity issues have many many root causes. Please file a separate JIRA ticket and provide more details about your project. Because we build swift-tools-support-core in CI in order to build the package manager, I believe your project has many other factors in play that would cause this diagnostic.