An Implementation Model for Rational Protocol Conformance Behavior

Evolution Pitches
37

I'm not sure it's really sound (enough) to treat differently-conforming Xs as the same type, because you can't assume the importance of the conformance has been captured in any enclosing generic type's generic parameter constraints. I'll try to make an example:

// Module A
extension Array where Element: Equatable {
   /// Returns true iff `self` contains any duplicate elements.
   /// - Complexity: O(n^2) where n == `count`
   func containsDuplicates() -> Bool { ... }
}

struct X {}

// Module B
import A
extension X: Equatable { 
  static func == (a: X, b: X) -> Bool {…}
}
/// Some Xs with no duplicate values, per equality as defined here.
public let uniqueXs: [X] = Array(Set( … ))

// Module C
import A
extension X: Equatable { 
  static func == (a: X, b: X) -> Bool {…} // Different equality
}

extension Array where Element: Equatable {
   /// - Requires: `!self.containsDuplicates()`
   func doSomething() { precondition(!self.containsDuplicates()) }
}

// Module D
import A
import B
import C

print(B.uniqueXs.containsDuplicates()) // true?
B.uniqueXs.doSomething()               // crash?
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