It doesn't crash for me on the master branch, where I get the following warning:

typealias overriding associated type 'Field' from protocol 'VectorSpace' is better expressed as same-type constraint on the protocol

Even in the version without the typealias? I'm seeing a crash in irgen::emitArchetypeWitnessTableRef, but admittedly I haven't pulled from master recently.

@swift-ci create

Comment by Maximilian Hünenberger (JIRA)

I just tested on Xcode 11.0 beta 4 and it still produces the segmentation fault.

Sorry, I tried with `-emit-ir` and both examples didn't cause a segfault, but yeah, when compiling normally it crashes in `emitArchetypeWitnessTableRef` as you mentioned. Sorry for the false alarm!

Comment by Maximilian Hünenberger (JIRA)

I tested it again on Xcode 11.0 beta 5 and it still produces a "Segmentation fault: 11" and after a clean build additionally "Command CompileSwift failed with a nonzero exit code".

I've been hitting a similar (same?) crasher for months now and finally had a few cycles to narrow it down. My case only happens if there are multiple source files, though, so it might not be exactly the same (still crashes in `emitArchetypeWitnessTableRef`.

This crashes for me still in Xcode 11.0 GM2 (11A420a).

In one file:

protocol Snapshotting {
    associatedtype NativeType: NativeInserting where NativeType.SnapshotType == Self
    associatedtype ChangeType: SnapshotChange where ChangeType.SnapshotType == Self
}

protocol NativeInserting {
    associatedtype SnapshotType : Snapshotting where SnapshotType.NativeType == Self
}

protocol SnapshotProperties : OptionSet where RawValue == Int {
    static var all: Self { get }
}

protocol SnapshotChange {
    associatedtype SnapshotType : Snapshotting where SnapshotType.ChangeType == Self
    associatedtype PropertiesType : SnapshotProperties
}

and in the other file:

class WatchRegistry {
    func single<S: Snapshotting>(objectType: S.Type) throws -> Watch<S>
    {
        return try Watch<S>.singleObject(objectType: S.self, properties: S.ChangeType.PropertiesType.all)
    }
}

class Watch<SnapshotType : Snapshotting> {
    static func singleObject(objectType: SnapshotType.Type, properties: SnapshotType.ChangeType.PropertiesType) throws -> Watch<SnapshotType> {
        fatalError()
    }
}

The original crash is fixed by #36402 @tjw's test case appears to be a different issue. I can confirm it crashes in Swift 5.1 but works in Swift 5.2, so it was fixed by some other change along the way. I'll add a regression test to be sure.