Why does Hashable require Equatable?

Evolution Discussion
3

Yes, that's how Dictionary is implemented now. But my question is why it has to be in that way and isn't it too much berden on the user to make sure == and hashValue don't contradict each other.

I've experience the problem when I imported a 3rd party framework that happened to be implementing Equatable to the type of which I was using for a dictionary key. I implemented hashValue and == to the type, but the compiler didn't warn me there were two == functions to the type, and I needed to realize my == method wasn't called at runtime to figure out the bug.

Anyway, when two objects are not equal but have the same hashValue, what behavior should a user reasonably expect?

struct Foo { let n }
extension Foo: Equatable {
  static func == (lhs: Foo, rhs: Foo) -> Bool { return rhs.n == lhs.n }
}
extension Foo: Hashable {
  var hashValue: Int { return n / 2 }
}

var dict1 = [Foo: Int]()
dict1[Foo(n: 2)] = 2
dict1[Foo(n: 3)] = 3
print("\(dict1.count)")  // I was expecting this to be 1.