ZeroDefaultingFail.swift

import Foundation

@propertyWrapper
struct ZeroDefaulting: Codable {
  var wrappedValue: Int?

  init(wrappedValue: Int?) {
    print("set:", wrappedValue ?? "nil")
    self.wrappedValue = wrappedValue
  }

  init(from decoder: Decoder) throws {
    self.wrappedValue = (try? Int(from: decoder)) ?? 0
    print("decoded:", wrappedValue!)
  }
}

struct Foo: Codable {
  @ZeroDefaulting var bar: Int!
}

let data = "{}".data(using: .utf8)!
let foo = try! JSONDecoder().decode(Foo.self, from: data)

print(foo.bar)