[SR-5853] Properties with a protocol type that conforms to Codable cannot be synthesized for Codable conformance

[swift-ci]

Previous ID	SR-5853
Radar	None
Original Reporter	fruitcoder (JIRA User)
Type	Bug
Status	Resolved
Resolution	Won't Do

Attachment: Download

Environment

Version 9.0 beta 6 (9M214v)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 7780a4010644f566517ae7386749b6a0

Issue Description:

protocol CodableConforming: Codable {}

struct TestStruct: Codable {
  let protocolProp: CodableConforming
}

When a type that conforms to `Codable` has a property whose type is a protocol conforming to `Codable`, the compiler cannot generate an implementation for `Encodable` and `Decodable`.

Moreover, I cannot provide the implementation myself, since the protocol type is insufficient for the generic `decode(:forKey: )` and `encode(:forKey: )` as I get "ambiguous reference to member" errors.

[belkadan]

That's correct. If you think about it, there's no way to _de_code this struct, since we don't know what concrete type to decode protocolProp with.

[swift-ci]

Comment by Alexander Hüllmandel (JIRA)

Of course, the only way to make this work would be to have a fully qualified type by adding the `CodableConforming` type to the `TestStruct`'s type as a generic parameter:

protocol CodableConforming: Codable {}

struct TestStruct<T: CodableConforming>: Codable {
  let protocolProp: T
}