[augmentation-libraries] Cover success case for aliased augmenting declarations

[eernstg]

PR #2568 contains a test LanguageFeatures/Augmentation-libraries/augmenting_types_A01_t02.dart checking that we get the expected compile-time errors in the case where a type alias of a type declaration of a mismatched kind is augmented.

Do we have a test which is testing the same situation, except that the type aliases denote types of the same kind? (That is, the case where we should not get an error because there is no kind mismatch, but there might still be an error because a tool mistakenly treats the type alias as a new and different kind and fails to see that the type alias denotes a declaration of the right kind).

[scheglov]

Where the fact that type aliases can be used is described in the specification?

[eernstg]

Ah, you're right, @scheglov, it is not supported that an augmenting declaration can use a type alias to refer to the augmented declaration. So this should be an error:

// --- Library 'main.dart'.
import augment 'augment.dart';

class B {}
class C {}
typedef CAlias = C;

// --- Library augmentation 'augment.dart'.
augment library 'main.dart';

augment class CAlias implements B {}

@sgrekhov, this implies that the tests introduced by #2581 need to be adjusted to expect errors: It is an error to augment class CAlias, but it is not because CAlias is an alias for something which isn't a class, it's an error because CAlias is itself a type alias and not a class.

This also implies that the meaning of the tests added in #2568, in particular LanguageFeatures/Augmentation-libraries/augmenting_types_A01_t02.dart and its augmentations, is different from what I was considering when reviewing them. (So augment mixin CAlias {} is still an error, but it isn't because CAlias is an alias of a class rather than an alias of a mixin, it's because CAlias is a type alias, full stop.) It is probably a good idea to take a look at the @description of these tests, and check that they do not convey anything which is wrong.

[sgrekhov]

Reopened

[sgrekhov]

Interesting. @eernstg specification contains the following syntax

typeAlias ::= 'augment'? 'typedef' typeIdentifier typeParameters? '=' type ';'
  | 'augment'? 'typedef' functionTypeAlias

What exactly can be augmented here? Could you show any useful example? We cannot change typeParemeters section, cannot change type, cannot change anything in functionTypeAlias

[sgrekhov]

Interesting. @eernstg specification contains the following syntax

typeAlias ::= 'augment'? 'typedef' typeIdentifier typeParameters? '=' type ';'
  | 'augment'? 'typedef' functionTypeAlias

What exactly can be augmented here? Could you show any useful example? We cannot change typeParemeters section, cannot change type, cannot change anything in functionTypeAlias

Ah, it's for adding metadata and DocComments. Ok

[eernstg]

It is not unthinkable that a type alias augmentation could do something which is similar to variable declarations:

typedef F<X> = List<X>;
augment typedef F<X> = Map<augmented<X>, augmented<X>>; // means `Map<List<X>, List<X>>`.

I wouldn't recommend it, though. 😄

So let's stick to the DartDoc comments and metadata.