Swift Init does not conform to expected type 'Decoder'

Question

At the moment, I have a struct that conforms to Codable:

public struct Preference: Codable {

    public let id: String

}

When I try to initialize the object using the following:

let preference = Preference(id: "cool")

I get the following error:

Argument type 'String' does not conform to expected type 'Decoder'

How can I fix this issue and initialize the struct properly?

I also can't reproduce this. I suspect this is not the actual code. What line does this error appear on? Is `Preference` have exactly this one property in your code? — Rob Napier, Jan 09 '18 at 19:31
@RobNapier This is actual code and the error occurs when trying to initialize the object. — Adam Cooper, Jan 09 '18 at 19:37
Perhaps you autocompleted to `let preference = Preference(from: "cool")` ? That would give you that error. Maybe you forgot to change it to the correct initializer or changed it and the compiler is slow in catching up? Maybe try a clean? — ekreloff, Jan 09 '18 at 19:38
@ekreloff if you use the "from:" initializer you've got to then force cast the string to conform to decoder. — Adam Cooper, Jan 09 '18 at 19:53
This compiles and runs correctly. Must be something else in your codebase? Xcode version? — picciano, Jan 09 '18 at 20:21
@AdamCooper, I'm just suggesting the circumstance when that error would appear. — ekreloff, Jan 10 '18 at 02:41
So apparently the type is defined in a framework (module), and the constructor called from outside of the module? – That would be an important information to add to the question. — Martin R, Jan 10 '18 at 08:09

Maxim Kosov · Accepted Answer · 2021-03-18T09:10:27.473

When struct is created without explicit initializer

public struct Preference {
    public let id: String
}

it gets internal init(id: String) initializer for free. Internal means that using it from another target will result in compiler error.

Adding Decodable to your struct also adds public init(from: Decoder) initializer to this struct.

So initial struct is equivalent to the following

public struct Preference: Codable {
    public let id: String

    internal init(id: String) {
        self.id = id
    }

    public init(from: Decoder) {
        // generated decoding code
    }
}

When you try to create an instance using Preference(id: "cool") from another target there is only one public initializer: the one with the decoder. Compiler tries to use it by casting String to Decoder and it fails.

To resolve the original issue you need to add public init(id: String) initializer explicitly.

Swift Init does not conform to expected type 'Decoder'

1 Answers1