[Pitch] Associated Type Disambiguation

[Slava_Pestov]

@vitamin

Ideally we could find a way in these cases where the same-named associated types are different, to detect the ambiguity, up-front, and emit an error—but from what I could tell, "detect and emit error instead of collapsing the types" cannot be layered on top of today's behavior. It seems to require replacing the merging behavior, which as you implied reaches pretty far down, and seemed like it would have ABI/source impact.

The right solution is to reject substitution maps with incompatible conformances at the call site, eg:

protocol P {
  associatedtype A
}

protocol Q {
  associatedtype A
}

struct G<T> {}

extension G: P where T: Hashable {
  typealias A = Bool
}

extension G: Q where T: Comparable {
  typealias A = Float
}

func f<T: P & Q>(_: T) {}

// all declarations up until here are good!

// but Int conforms to both Hashable and Comparable:

f(G<Int>())  // so an error must be diagnosed here

This is a known issue that I'd like to fix some day. However, it's a fairly involved fix that requires a good understanding of Part 4 in https://download.swift.org/docs/assets/generics.pdf, as well as the requirement minimization algorithm, which is not even documented yet.