Why can't I cast `Woo<Any>` to `Woo<Int>` concrete type? But `Array<Any>` to `Array<Int>` is OK?

Using Swift
16

If you'd like to stop

xs2.append("yello")

from being possible in that example, I think that what you'd really want is some Array or any Array, not [Any]. You can see the underpinnings of the restrictions that would be applied when Element is missing:

var xs2: some RangeReplaceableCollection = xs1
xs2.append(4) // Cannot convert value of type 'Int' to expected argument type '(some RangeReplaceableCollection).Element'

var xs2: any RangeReplaceableCollection = xs1
xs2.append(4) // No exact matches in call to instance method 'append'
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