protocol P {
    associatedtype E = String
    
    func foo1(_ arg: E)
    func foo2(_ arg: E)
}
 
protocol P1: P \{...} // (*) See bottom line
 
class Foo: P {
    func foo1(_ arg: E) {}
    func foo2(_ arg: E) {}
}
 
extension P \{ typealias E = Bool } // This will break Foo...
// ...and any conformances down the whole protocol hierarchy
 
// (*) All further occurrences of E will refer to the typealias instead.

protocol P {
    associatedtype A = String
    
    func foo(_ arg: A)
}
 
class Foo: P {
    func foo(_ arg: A) {}
}
 
extension Foo {
    typealias A = Bool
}
// No redeclaration error and conformance is broken