[swift-evolution] 'T != Type' in where clause

[Robert Bennett]

I'm also annoyed by this, so I second this proposal.

To give an explicit example of the kind of code that can lead to frustration and a desire for "T != Type":

protocol P1 {}

protocol P2 {
	// Now, any type conforming to P1 and P2 may not be added to itself
	static func +<T: P1>(lhs: Self, rhs: T) -> Self
	static func +<T: P1>(lhs: T, rhs: Self) -> Self
}

struct S: P1, P2 {
	static func +<T: P1>(lhs: S, rhs: T) -> S { return lhs }
	static func +<T: P1>(lhs: T, rhs: S) -> S { return rhs }
}

let s1 = S()
let s2 = S()

print(s1 + s2) // ambiguous use of operator '+'


If I understand, this would be the desired capability:

protocol P1 {}

protocol P2 {
	// Now, there's no ambiguity
	static func +<T: P1 where T != Self>(lhs: Self, rhs: T) -> Self
	static func +<T: P1 where T != Self>(lhs: T, rhs: Self) -> Self
	static func +(lhs: Self, rhs: Self) -> Self
}

struct S: P1, P2 {
	static func +<T: P1 where T != Self>(lhs: T, rhs: S) -> S {
		return rhs
	}
	static func +<T: P1 where T != Self>(lhs: S, rhs: T) -> S {
		return lhs
	}
	static func +(lhs: S, rhs: S) -> S {
		return lhs
	}
}

Some thought would have to be given to how to handle subtype relationships — probably you would use the polymorphic type of the object at the call site.