Pitch: Genericizing over annotations like throws

Evolution Pitches
10

Another interesting future direction we could take to address polymorphism over throws specifically would be to adopt typed throws, and make the error type an independent type argument of all function types. This would mean that a type that doesn't throw effectively throws Never, and one that throws without a type by default throws Error:

(X, Y, Z) -> W        === (X, Y, Z) throws Never -> W
(X, Y, Z) throws -> W === (X, Y, Z) throws Error -> W

This would allow protocols to be generic over throwing and nonthrowing implementations by making the error a separate associated type:

protocol MyIterator {
  associatedtype Element
  associatedtype Error: Swift.Error

  mutating func next() throws Error -> Element
}

struct NonthrowingImplementation: MyIterator {
  mutating func next() -> Int // conforms with Error == Never
}
struct ThrowingImplementation: MyIterator {
  mutating func next() throws -> Int // conforms with Error == Swift.Error
}
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