protocol P {
    associatedtype A
    typealias B = Int
}
extension P where A == Bool {
    typealias B = Float // Compiles, but should it?
}
extension P where A == Double {
    typealias B = String // ERROR: Invalid redeclaration of `B`
}
// Note that Bool, Int, Float, Double and String can be any types; ie
// the topmost extension will always compile and the rest will not.
// Same behavior with generic typealiases.
// Should both compile (thus shadowing the default B) or should both be
// considered redeclarations?

// The same example but with a struct instead of a protocol consistently
// treats both as redeclarations:
struct S<A> {
    typealias B = Int
}
extension S where A == Bool {
    typealias B = Float // ERROR: Invalid redeclaration of `B`
}
extension S where A == Double {
    typealias B = String // ERROR: Invalid redeclaration of `B`
}