Open Radar

 
The code below yields the error shown. I accept that the call might be ambiguous or even might yield the unexpected result of a Test instance. However, the actual error message complains of being unable to find any valid interpretation, which seems wrong. The supplied expression should either be a Double or a Test, both of which have abs() defined. Nu?

protocol TestConvertible {
    func toTest() -> Test
}

class Test: TestConvertible {
    func toTest() -> Test { return self }
}

extension Double: TestConvertible {
    func toTest() -> Test { return Test() }
}

func abs(x: Test) -> Test {
    return Test()
}

func -<T: TestConvertible, U: TestConvertible>(lhs: T, rhs: U) -> Test {
    return Test()
}

let aNumber = 23.4 - 3.1415     // aNumber is Double
let abs1 = abs(aNumber)         // Fine, 20.26
let abs2 = abs(23.4 - 3.1415)   // Error: could not find an overload for 'abs'
                                //   that accepts the supplied arguments