Separate decoding process into normal and custom. See Also: https://github.com/vincent-pradeilles/swift-tips#implementing-pseudo-inheritance-between-structs

NotAllCodingKeys.swift

import Foundation

let intJson = #"{ "inUse": 1, "name": "Daiki Matsudate", "twitter": "d_date", "stars": 99999, "occupation": null}"#
let boolJson = #"{ "inUse": true, "name": "Daiki Matsudate", "twitter": "d_date", "stars": 99999, "occupation": null}"#

protocol Inherits {
    associatedtype SuperType

    var `super`: SuperType { get }
}

extension Inherits {
    subscript<T>(dynamicMember keyPath: KeyPath<SuperType, T>) -> T {
        return self.`super`[keyPath: keyPath]
    }
}

@dynamicMemberLookup
struct A: Decodable, Inherits {
    enum CodingKeys: String, CodingKey {
        case inUse
    }

    let `super`: SuperA
    let inUse: Bool


    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.super = try .init(from: decoder)
        self.inUse = {
            if let boolValue = try? container.decode(Bool.self, forKey: .inUse) {
                return boolValue
            } else if let intValue = try? container.decode(Int.self, forKey: .inUse) {
                return NSNumber(value: intValue).boolValue
            }
            fatalError()
        }()
    }
}

struct SuperA: Codable {
    let name: String
    let twitter: String
    let stars: Int
    let occupation: String?
}

let decoder = JSONDecoder()
let fromInt = try decoder.decode(A.self, from: intJson.data(using: .utf8)!)
let fromBool = try decoder.decode(A.self, from: boolJson.data(using: .utf8)!)
print("twitter", fromInt.twitter, separator: ": ")
print("occupation", fromInt.occupation ?? "無職", separator: ": ")

He raised issue about json decoding first:
https://twitter.com/shunkun_san/status/1189091946695757829?s=20