Storing a ShapeStyle in a view?

I have a ProgressBar custom view, and I would like the ability to set a custom fill on it. I can figure out how to do this with a color, but I want to be able to specify either a color or a gradient.

The problem is that ShapeStyle has an associated type, and there is no TypeErased version that I know of, so I have no way to store the fill.

Is there something I am missing, or a better way to do this?

struct ProgressBar<S:Shape> : View {
    var percent:CGFloat
    var shape:S
    
    init(shape:S, percent:CGFloat) {
        self.percent = percent
        self.shape = shape
    }
    
    var body: some View {
        ZStack(alignment: .leading) {
            Rectangle().fill().relativeSize(width: percent, height: 1.0).clipShape(shape)
            shape.stroke()
        }
    }
}

extension ProgressBar where S == RoundedRectangle {
    init(corner:Length, percent:CGFloat) {
        self.shape = RoundedRectangle(cornerRadius: corner)
        self.percent = percent
    }
}

extension ProgressBar where S == Rectangle {
    init(percent: CGFloat) {
        self.shape = Rectangle()
        self.percent = percent
    }
    
    func cornerRadius(_ radius: Length) -> ProgressBar<RoundedRectangle> {
        return ProgressBar<RoundedRectangle>(corner: radius, percent: self.percent)
    }
}