@swift-ci create

Following the substitutions here and replacing

func b(_ b: T.B) -> B { fatalError() }

By

func b(_ b: (S1.B, S2.B)) -> B { fatalError() }

Appears to work around this. I still do not have a root cause.

It appears the conformance of S1 and S2 to SP is invalid because the type requirement `B` is invalid. Another, independent, workaround

extension S1: SP {
public typealias B = LS<[String: M1], String>

public var b: B { fatalError() }
}
extension S2: SP {
public typealias B = LS<[N.M2], String>

public var b: B { fatalError() }
}

Comment by Matthew Johnson (JIRA)

Thank you for the update with a workaround!

`protocol SP: SS` should also not attempt to redeclare the type requirements A, B, and C. That won't buy you anything.

Comment by Matthew Johnson (JIRA)

IIRC I have seen the redeclaration improve the compiler's ability to infer associated types. It may be that this was true in the past and is no longer true. I agree that it shouldn't be necessary to redeclare them.

I believe in this case it's actually a cause of the instability. It creates aliases to these type requirements, and when one of those aliases fails then substitutions involving it propagate forward silently. It's possible those aliases are enabling the extensions you have to be picked up, though, in which case it's possible this compiled in 5.1 by accident.