function-local struct def can never be Comparable?

If i have a struct inside a function, swift keeps telling me is doesn't conform to Comparable, even though the same definition is fine outside of a function -- why is this?

Foo fails Comparable here:

func foo() {
            struct Foo : Equatable, Comparable {
                let x: Int
                let y: Int
                static func < (lhs: Foo, rhs: Foo) -> Bool {
                    return lhs.x+lhs.y < rhs.x+rhs.y
                }
            }
        }