1

I found it is compiled failed for property wrappers with codable in multiple files.

I found test codes in Swift source below:

@propertyWrapper
struct Printed<Value: Codable>: Codable {
    var wrappedValue: Value {
        didSet { print(wrappedValue) }
    }
}

struct Foo: Codable {
    @Printed var bar: Bool = false
}

func test(_ value: Foo = Foo()) {
  let _: Codable = value
}

and use them in my test project:

TestProject

But it is compiled failed with error:

Type 'Foo' does not conform to protocol 'Encodable'

How to fix it?

2

I found that XCode compiled successfully without codable, but compiled failed using codable.

Compile successfully:

    func test() {
        let value = Foo()
        print("test: \(value)")
        
//        // Using codable cause compilation error;
//        if let data = try? JSONEncoder().encode(value),
//            let json = String.init(data: data, encoding: .utf8) {
//            print(json)
//        }
    }

Compile failed:

    func test() {
        let value = Foo()
        print("test: \(value)")
        
        // Using codable cause compilation error;
        if let data = try? JSONEncoder().encode(value),
            let json = String.init(data: data, encoding: .utf8) {
            print(json)
        }
    }

Is it because property wrappers with codable generates codable code after compilation, but the JSONEncoder need the "Foo" struct conforms to codable protocol before compilation.

3

Looks like a bug, esp. that it is fixed if you move everything into 1 file. Perhaps you can file bug report.

4

Everything in one file is OK. I have filed a bug for XCode in https://feedbackassistant.apple.com/. I don't know it is a bug for XCode team or Swift team.

5

Can you try removing Codable conformance from the wrapper itself?

6

It is compiled failed when removing codable from wrapper itself. Because the struct "Foo" does not conform to codable protocol.