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Warning: Cast from `(T, T)` to unrelated type `(Float, Float)` always fails

Occurs when using pattern matching to check type of two items using a tuple.  The casting works as expected and does not always fail, so the warning is incorrect.  

Code below shows example case:

func SumNums<T:NumConvertible>( lhs:T,_ rhs:T) -> T {
    switch (lhs,rhs) {
        case let x as (Float, Float) :
            return T(x.0 + x.1)
        case let x as (Double, Double) :
            return T(x.0 + x.1)
        default:
            assert(false, "NumConvertible convert cast failed!")
            return T(0.0)
    }
}

public protocol NumConvertible:Equatable,Comparable{
    init(_ value: Float)
    init(_ value: Double)
}