[analyzer][Extension types] No error when extension type has a member conflict

[sgrekhov]

Test https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Extension-types/superinterfaces_of_extension_type_A05_t08.dart works in CFE but fails in analyzer. The code is

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

extension type V(int id) {
  external int id2;
}

extension type ET(int id) implements V {
  int id2() => id; // No expected error in analyzer
//    ^^^
// [analyzer] unspecified
// [cfe] unspecified
}

main() {
  print(ET);
}

Tested on Dart SDK version: 3.3.0-edge.e2aa60ddf362967e17e06ec08871d634a32832c1 (be) (Wed Nov 1 04:17:41 2023 +0000) on "macos_x64"

[scheglov]

Ah, I opened dart-lang/co19#2354
I don't understand why this is a compile-time error.
@eernstg

[eernstg]

Actually, I also can't see any conflicts: ET has a declaration that induces a getter int get id and a method int id2(), and it inherits nothing (because V has a member named id and a member named id2, and they are both redeclared by ET).

The failure I can see (but I haven't looked into running the test on many configurations) is that the DartPad compiler reports an error on external int id;. But that's simply because this is an external getter which is not associated with any implementation (I think it's enough for the representation type to be JSAny or a subtype thereof, but the compiler needs something in order to know how to bind that getter to an actual external implementation), and hence it is rejected by the compiler.

@sgrekhov, what would be conflict be?

[sgrekhov]

Please see #53901 (comment)
As far as I understand the key point here is that external int id2; is not final here and therefore introduces a setter as well. According to dart spec

It is a compile-time error if the interface of C has an instance method named n and an instance setter with basename n.

[eernstg]

Ah, of course, external int id2 does introduce a setter named id2=, and we can't have that in the interface of ET together with a method named id2. Thanks!

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/334640