Can extension types declare abstract members?

[srawlins]

I mean, I'm pretty sure the answer is "no," but I don't see this explicitly specified, and it probably should be.

FWIW, the static extension methods spec explicitly states:

It is a compile-time error if an extension [...] declares an abstract member.

So I think it's worth it to specify this, add tests, etc.

(Aaand I think native and external methods should be allowed?)

CC @eernstg

[eernstg]

It is a compile-time error if an extension [...] declares an abstract member.

Yes, extension types should have the same rule. Thanks!

[eernstg]

This change is now included in #3276.

[eernstg]

Cf. dart-lang/co19#2216.

[rrousselGit]

In a world with macros, would it make sense?

It could initially be declared abstract, to be later augmented

[eernstg]

We have a proposal saying that an abstract member in an extension type declaration can be used for a different purpose: It is allowed if it is a member declaration for which a member of the representation type is a correct override. In other words, it serves as a request for automatic generation of a forwarding method that calls a similar method on the representation object.

class A {
  String foo(num n) {...}
}

extension type E1(A a) {
  String? foo(int i); // Implicitly becomes `String? foo(int i) => a.foo(i);`
}

This proposal fits rather well with the fact that abstract members are an error at this time, because we can then use them for this purpose (and/or other purposes) later on.

With respect to macros: It is probably no different from abstract members of other declarations. For example, if we wish to use a macro to provide the implementation of a member declaration which is specified as an abstract member of a class then we would probably also want to allow that abstract member to exist in the original source code even though the class is non-abstract. In other words, it is no more of a problem to use abstract members in this way with extension types than it is with classes, so the solution from classes should also work for extension types.