cannot return reference iwth incompatible origin

[odd shadow]

What's the easiest way to deal with this error?

[abstract gorge]

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[odd shadow]

This is the whole code snippet:

struct Shape:
    var area: Float64


trait Drawable:
    fn get_shapes(ref self) -> ref [self] List[Shape]:
        ...

@value
struct Canvas(Drawable):
    var shapes: List[Shape]

    fn get_shapes(ref self) -> ref [self] List[Shape]:
        return self.shapes

    fn total_area(self) -> Float64:
        var total: Float64 = 0
        for shape in self.shapes:
            total += shape[].area
        return total

fn main():
    var canvas = Canvas(shapes=List[Shape]())

    shapes = canvas.get_shapes()
    shapes.append(Shape(area=10.5))
    print(canvas.total_area())

As there is a Drawable trait that does not know the shapes attribute, we cannot just replace ref [self] with ref [self.shapes]