[Bug] superDecoder(forKey:) doesn’t work as documented

[soumyamahunt]

When trying to decode a Codable model with following implementation:

struct SomeCodable {
    let value1: String?
}

extension SomeCodable: Decodable {
    init(from decoder: any Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        if let key1_containerDecoder = try? container.superDecoder(forKey: CodingKeys.key1) {
            let key1_container = try key1_containerDecoder.container(keyedBy: CodingKeys.self)
            self.value1 = try key1_container.decodeIfPresent(String.self, forKey: CodingKeys.value1)
        } else {
            self.value1 = nil
        }
    }
}

extension SomeCodable: Encodable {
    func encode(to encoder: any Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        var key1_container = container.nestedContainer(keyedBy: CodingKeys.self, forKey: CodingKeys.key1)
        try key1_container.encodeIfPresent(self.value1, forKey: CodingKeys.value1)
    }
}

extension SomeCodable {
    enum CodingKeys: String, CodingKey {
        case value1 = "key2"
        case key1 = "key1"
    }
}

and empty JSON object({}) error is thrown at

let key1_container = try key1_containerDecoder.container(keyedBy: CodingKeys.self)

valueNotFound(Swift.KeyedDecodingContainer<MetaCodableTests.SomeCodable.CodingKeys>, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "key1", intValue: nil)], debugDescription: "Cannot get keyed decoding container -- found null value instead.", underlyingError: nil))"

instead of

if let key1_containerDecoder = try? container.superDecoder(forKey: CodingKeys.key1) 

which is diferrent than documented.

As per discussion in forum, both older and new Foundation implementation seems to have this bug.