Private conformance to public protocol without making internal members / methods public.

private-conformance-to-public-protocol.swift

// Private Conformance (Generic Manifesto)
// https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#private-conformances
//
// > Right now, a protocol conformance can be no less visible than the minimum of the conforming type's access and the protocol's access.
// > Therefore, a public type conforming to a public protocol must provide the conformance publicly.
// > The main problem with private conformances is the interaction with dynamic casting

public protocol PublicProtocol {
    var foo: String { get }
}

// OK
internal struct Struct: PublicProtocol {
    internal var foo: String = ""
}

// ERROR: Property 'foo' must be declared public because it matches a requirement in public protocol 'PublicProtocol'
//public struct PublicStruct: PublicProtocol {
//    fileprivate var foo: String = ""
//}

// ERROR: This doesn' work too.
//internal extension PublicStruct: PublicProtocol {
//    var foo: String { "" }
//}

// Goal: always keep `foo` as private, but conform to `PublicProtocol` for internal usage only.
public struct PublicStruct {
    internal var foo: String = ""
}

// Solution:
// Add **internal proxy** that conforms to `PublicProtocol` instead.
// Inside the same module, use this type to gain `PublicProtocol` behavior.
// For the outside of the module, `PublicStruct` is the only public as usual.
internal struct PrivateProxy: PublicProtocol {
    internal var inner: PublicStruct

    // Forward all protocol requirements... 
    internal var foo: String { // But this is still exposing `foo`...
        inner.foo
    }
}