[SR-5833] Cannot convert non-generic subtype of generic type

[ffried]

Previous ID	SR-5833
Radar	None
Original Reporter	@ffried
Type	Improvement

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Compiler | |Labels | Improvement, LanguageFeatureRequest | |Assignee | None | |Priority | Medium | md5: 66830de8198992693ebd5902cb0e0057

Issue Description:

Given the following code:

class Base<Some> {
    enum Kind { case a, b }

    init(kind: Kind) {}
}
class SubclassA: Base<Int> {}
class SubclassB: Base<String> {}

let kind = Base<Int>.Kind.a
_ = SubclassA(kind: kind)
_ = SubclassB(kind: kind)

The compiler emits the following:

s4_nongeneric_subtypes.swift:11:21: error: cannot convert value of type 'Base<Int>.Kind' to expected argument type 'Base<String>.Kind'
_ = SubclassB(kind: kind)
                    ^~~~

While this is basically correct, the nested enum Kind does not use the generic types of the outer type. So theoretically it could use used interchangeably.
Furthermore, accessing the nested type always requires specifying the generic type. E.g. let kind = Base.Kind.a results in an error.

It would be great if nested types, that don't reference the generic types of their outer type could be used interchangeably.

As a workaround, the following works:

extension Base.Kind {
   func toKind<T>() -> Base<T>.Kind {
      switch self {
         case .a: return .a
         case .b: return .b
      }
   }
}

let kind = Base<Int>.Kind.a
_ = SubclassA(kind: kind)
_ = SubclassB(kind: kind.toKind())

[belkadan]

This would be a language change—although you're right that it's a relatively esoteric one—and would thus have to go through the Swift Evolution Process.