This is a workaround for the "Protocol 'Encodable' as a type cannot conform to the protocol itself" paradox. It provides a way to explicitly open up existential Encodable types and encode them using an encoder.

example usage.swift

import Foundation
let someEncodableThing: Encodable = "Hello world"

#if swift(>=5.7)
  let data = try JSONEncoder().encode(json)
#else
  let data = try JSONEncoder().encode(existential: json)
#endif

Explicitly Opened Existentials.swift

//
//  Encode Existentials.swift
//
//  This is a workaround for the "Protocol 'Encodable' as a type cannot conform to the protocol itself" paradox.
//  All the code in this file can be safely removed after upgrading to the swift 5.7 compiler.
//  Calls to ``TopLevelEncoder/encode(existential: Encodable)`` can then safely be replaced
//  with a call to the encoder's `encode(_:)` function
//  due to the "Implicitly Opened Existentials" (SE-0352) feature implemented in swift 5.7: https://github.com/apple/swift-evolution/blob/main/proposals/0352-implicit-open-existentials.md
//
//  Created by Damiaan Dufaux on 09/06/2022.
//

#if swift(<5.7)
import class Foundation.JSONEncoder
extension JSONEncoder: TopLevelEncoder {}

/// A type that defines methods for encoding.
protocol TopLevelEncoder {
    /// The type this encoder produces.
    associatedtype Output

    /// Encodes an instance of the indicated type.
    ///
    /// - Parameter value: The instance to encode.
    func encode<T>(_ value: T) throws -> Self.Output where T : Encodable
}

extension TopLevelEncoder {
    /// Explicitly opens an existential `Encodable` type and encodes it.
    func encode(existential: Encodable) throws -> Output {
        try existential.encode(using: self)
    }
}

extension Encodable {
    /// Explicitly opens a potential existential Encodable and encodes it using the specified TopLevelEncoder.
    func encode<E: TopLevelEncoder>(using encoder: E) throws -> E.Output {
        // `self` here, is a reference to the concrete 'underlying' type of a potential existential type.
        try encoder.encode(self)
    }
}
#endif