Can a Codable protocol be used for decoding without exposing the underlying type?

Question

I want to do something like this where the underlying concrete types are not used, and instead only a Codable protocol is used.

protocol MyProtocol: Codable {}

struct MyProtocolImpl: MyProtocol {}

struct Something: Codable {
    let x: MyProtocol
}

This does not compile with:

Type 'Something' does not conform to protocol 'Decodable'

I assume this is intentional by Swift, but I want to check whether I am missing something.