Not just Swift 5.1, but they also require iOS 13 because there's a rather unfortunate stdlib dependency. Technically they only need the dependency when crossing

Not just Swift 5.1, but they also require iOS 13 because there's a rather unfortunate stdlib dependency. Technically they only need the dependency when crossing module boundaries, but they're restricting the feature completely anyway, I guess because they don't want to bother explaining "you can use 'some' on internal APIs but you can't use it on a public API without iOS 13+".