[SR-9599] CodingKeys are improperly generated for nested Decodable/Encodable objects

[swift-ci]

Previous ID	SR-9599
Radar	None
Original Reporter	jonstaff (JIRA User)
Type	Bug

Additional Detail from JIRA

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

md5: 905f4d7055e6d21048c58a6d805c48d7

Issue Description:

Referencing the compiler-generated CodingKeys enum within the init of a nested Decodable or Encodable fails. For example:

struct Outer: Decodable {
  var a: Int

  struct Inner: Decodable {
    var b: Int

    init(from decoder: Decoder) throws {
      // CodingKeys should be Inner.CodingKeys, but is Outer.CodingKeys
      let container = try decoder.container(keyedBy: CodingKeys.self)
      b = try container.decode(Int.self, forKey: .b) // compiler error
    }
  }
}

However, this works perfectly fine for objects conforming to Codable:

struct Outer: Codable {
  var a: Int

  struct Inner: Codable {
    var b: Int

    init(from decoder: Decoder) throws {
      // CodingKeys is Inner.CodingKeys
      let container = try decoder.container(keyedBy: CodingKeys.self)
      b = try container.decode(Int.self, forKey: .b)
    }
  }
}

When an explicitly-defined CodingKeys enum is provided, everything works as expected:

struct Outer: Decodable {
  var a: Int

  struct Inner: Decodable {
    var b: Int

    private enum CodingKeys: CodingKey {
      case b
    }

    init(from decoder: Decoder) throws {
      // CodingKeys is Inner.CodingKeys
      let container = try decoder.container(keyedBy: CodingKeys.self)
      b = try container.decode(Int.self, forKey: .b) // works fine
    }
  }
} 

Possibly related to https://bugs.swift.org/browse/SR-5215

[belkadan]

Hm. The way CodingKeys synthesis works is that it does a lookup first and only synthesizes a custom one if the look fails. In this case, that comes out to "lookup in Inner -> lookup in Outer -> synthesize in Outer" and we never get back to "synthesize in Inner". I think that means we should be doing a qualified lookup into the Self type, even though that's technically a breaking change.