is duplicated by:

• SR-7429 Cannot conform a non-final class to a protocol with a defaulted requirement with a generic placeholder constrained to an associated type
• SR-7435 Compiler incorrectly disallows nonfinal classes from using default implementations that have same-type constraints on associated types of Self

relates to:

• SR-7429 Cannot conform a non-final class to a protocol with a defaulted requirement with a generic placeholder constrained to an associated type

Issue Description:

We have a simple protocol with an associated type and one method

protocol Node {
    associatedtype ValueType
    
    func addChild<ChildType>(_ child: ChildType)
         where ChildType: Node, ChildType.ValueType == Self.ValueType
}

a default implementation for that method via a protocol extension

extension Node {
    func addChild<ChildType>(_ child: ChildType) 
         where ChildType: Node, ChildType.ValueType == Self.ValueType
    {
        print("default implementation goes here")
    }
}

and a conforming class

class IntNode: Node {
    typealias ValueType = Int
}

This code compiled under both Swift 3.2 and 4.0 with the Swift tools bundled with Xcode 9.2. However, since updating to Xcode 9.3, the compiler produces the following error, under both Swift 3.3 and 4.1:

error: type 'IntNode' does not conform to protocol 'Node'
class IntNode: Node {

note: candidate has non-matching type '<Self, ChildType> (ChildType) -> ()'
    func addChild<ChildType>(_ child: ChildType) where ChildType: Node, ChildType.ValueType == Self.ValueType {

note: do you want to add protocol stubs?
class IntNode: Node {

This occurs regardless of the underlying platform (we tested on both iOS and MacOS), or whether we compile via Xcode or directly on the command line with swiftc.