Does the order of P1 and P2 matter in: extension P1 where Self: P2?

Using Swift
6

Here's the shortest example I could come up with while trying to reduce my real world scenario (which doesn't include Array or Collection):

protocol P {
    associatedtype A: Collection where A.Element == Self
}
extension Collection where Self: P {
    typealias A = Array<Self>
}

The above compiles successfully, but swapping Collection and P in the extension will result in compile time errors.

Details

collection_first.swift:

protocol P {
    associatedtype A: Collection where A.Element == Self
}
extension Collection where Self: P {
    typealias A = Array<Self>
}

p_first.swift:

protocol P {
    associatedtype A: Collection where A.Element == Self
}
extension P where Self: Collection {
    typealias A = Array<Self>
}

I can reproduce the behavior using both the default toolchain of Xcode 9.4 and the most recent snapshot:

› swiftc --version
Apple Swift version 4.1.2 (swiftlang-902.0.54 clang-902.0.39.2)
Target: x86_64-apple-darwin17.5.0
›
› /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2018-06-05-a.xctoolchain/usr/bin/swiftc --version
Apple Swift version 4.2-dev (LLVM 031e148970, Clang b58a7ad218, Swift 0747546bd7)
Target: x86_64-apple-darwin17.5.0

collection_first.swift compiles successfully:

› swiftc collection_first.swift 
› 
› /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2018-06-05-a.xctoolchain/usr/bin/swiftc collection_first.swift 
›

p_first.swift fails to compile with the following errors (which are slightly different for the two versions of the compiler):

› swiftc p_first.swift 
p_first.swift:2:40: error: type alias 'A' references itself
    associatedtype A: Collection where A.Element == Self
                                       ^
p_first.swift:5:15: note: type declared here
    typealias A = Array<Self>
              ^
p_first.swift:2:40: error: 'A' is ambiguous for type lookup in this context
    associatedtype A: Collection where A.Element == Self
                                       ^
p_first.swift:2:20: note: found this candidate
    associatedtype A: Collection where A.Element == Self
                   ^
p_first.swift:5:15: note: found this candidate
    typealias A = Array<Self>
              ^
p_first.swift:2:50: error: same-type requirement makes generic parameter 'Self' non-generic
    associatedtype A: Collection where A.Element == Self
                                                 ^
› 
›
› /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2018-06-05-a.xctoolchain/usr/bin/swiftc p_first.swift 
p_first.swift:2:40: error: type alias 'A' references itself
    associatedtype A: Collection where A.Element == Self
                                       ^
p_first.swift:5:15: note: type declared here
    typealias A = Array<Self>
              ^
p_first.swift:2:40: error: 'A' is ambiguous for type lookup in this context
    associatedtype A: Collection where A.Element == Self
                                       ^
p_first.swift:2:20: note: found this candidate
    associatedtype A: Collection where A.Element == Self
                   ^
p_first.swift:5:15: note: found this candidate
    typealias A = Array<Self>
              ^
›

Note that the same-type requirement is not necessary to reproduce the behavior, Self can be replaced with eg Int and the two variants of the program will still demonstrate this behavior.

This example is perhaps too reduced and contrived to show why I think there might be valid reasons rather than a compiler bug behind this. But I might well be wrong.

Maybe I can post a better example later, unless someone can bring clarity to this before that?