[SR-5610] Incorrect redundant same-type constraint warning related to recursive constraints

[jepers]

Previous ID	SR-5610
Radar	None
Original Reporter	@jepers
Type	Bug
Status	Closed
Resolution	Done

Environment

Xcode 9 beta 4, dev snapshot 2017-07-31

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	@DougGregor
Priority	Medium

md5: f0aea715c2c4dc6b1e1eb164db9de82a

Issue Description:

protocol P1 {
    associatedtype X: P2 where X.Q == Self, X.R == UInt8
}
struct A: P1 {
    typealias X = S<UInt8>
}
protocol P2 {
    associatedtype Q: P1
    associatedtype R
}
struct S<E> : P2 {
    typealias R = E
    typealias Q = A
}
extension P2 {
    func map<T>(transform: (R) -> T.R) -> T
        where T: P2, Q == T.Q // <-- Incorrect warning here:
        //     ^----------------------------------------------------------.
        // Redundant same-type constraint 'Self.Q' == 'T.Q'               |
        // Previous same-type constraint 'T.Q' == 'T.Q.X.Q' implied here -'
    {
        fatalError() // <-- Not important ...
    }
}

[huonw]

Reduced (I think, I'm temporarily Xcode-less, so can't be sure it behaves the same with the latest compiler):

protocol P1 {
  associatedtype X : P2 where X.Q == Self
}
protocol P2 {
  associatedtype Q : P1
  func map<T>(_: T.Type) where T : P2, Q == T.Q
}

(See also https://bugs.swift.org/browse/SR-5473 and https://bugs.swift.org/browse/SR-5601.)

Thanks for helping to shake out all the bugs here @jepers!

[jepers]

Ah, I can confirm that Huon's reduced version produces the same incorrect warning in dev snapshot 2017-07-31.

[DougGregor]

#11751