@dynamicMemberLookup via another @dynamicMemberLookup

Using Swift
1

On the example of AttributedString type it is not very clear how the dynamicMemberLookup chaining works from

@dynamicMemberLookup
struct AttributedString {
    subscript<K>(dynamicMember keyPath: Swift.KeyPath<AttributeDynamicLookup, K>) -> K.Value? where K: AttributedStringKey {
 // ...
    }
}

through

public extension AttributeDynamicLookup {
    subscript<T: AttributedStringKey>(dynamicMember keyPath: KeyPath<AttributeScopes.FoundationAttributes, T>) -> T {
        // ... <--- Not called!!!
    }
}

What's not clear here is that the second subscript is not called at all. How does it work?

var attr = AttributedString()
_ = attr.foo

// ...

extension AttributeDynamicLookup {
    subscript<T>(dynamicMember keyPath: KeyPath<AttributeScopes.MyAttributes, T>) -> T where T: AttributedStringKey {
        self[T.self] // <--- Not called!!!
    }
}

extension AttributeScopes {
    var my: AttributeScope.Type {
        MyAttributes.self
    }
    struct MyAttributes: AttributeScope {
        let foo: FooAttribute
    }
}

enum FooAttribute: AttributedStringKey {
    typealias Value = Int
    static let name = "MyAttribute"
}

// ...

@dynamicMemberLookup
struct AttributedString {
    subscript<K>(dynamicMember keyPath: Swift.KeyPath<AttributeDynamicLookup, K>) -> K.Value? where K: AttributedStringKey {
        self[K.self]
    }
    subscript<K>(_: K.Type) -> K.Value? where K: AttributedStringKey {
        // ...
    }
}

@dynamicMemberLookup
enum AttributeDynamicLookup {
    subscript<T>(_: T.Type) -> T where T: AttributedStringKey {
        fatalError()
    }
}

public enum AttributeScopes {
}

protocol AttributeScope {
}

public protocol AttributedStringKey {
    associatedtype Value: Hashable
    static var name: String { get }
}