[Extension types] Can extension type implement type Object?

[sgrekhov]

The following code works in analyzer but fails in CFE.

// SharedOptions=--enable-experiment=inline-class

extension type ET(Object id) implements Object {} // CFE: Error: The type 'Object' can't be implemented by an extension type.

main() {
  print(ET);
}

@eernstg which tool wins here?

[mkustermann]

/cc @lrhn

[eernstg]

We don't have any rules against implements Object that I can find, so that would make it a CFE issue.

[lrhn]

Agree. The types you just cannot implement are:

• void
• dynamic
• T? for any T
• FutureOr<T> for any T
• Function
• any function type
• Record
• any record type
• Null
• Never

That leaves only extension types and "interface types" (types of class, mixin or enum types) that are not mentioned above.

To implement any such type, T, the extension types with representation type R must satisfy at least one of the requirements:

• R <: T, or
• T is extension type with representation type R2 and R <: R2.

(Since the representation type now cannot be a bottom type, that also rules out implementing Never.)