Lint for merging equal import/exports with different show combinators

[FMorschel]

When you have duplicate import/exports like:

export 'lib1.dart' show A;
export 'lib1.dart' show B;

we could have a lint for merging them together.

Old description

Say you have a package (happened to mine - due_date). You have an API that should be exported separately outside the project in different libraries (one file with multiple declarations or a barrel file).

So you write down an export directive with show/hide. But later you add more things and you add another entry (forgot about the original one or didn't see it) with the same file but other show/hide values. This should be warned to make them a single export. Even more important if they are using the hide combinator since the other may show unwanted things.

export 'lib1.dart' show A;
export 'lib1.dart' show B;

export 'lib2.dart' hide C;
export 'lib2.dart' hide D;

Today if we have the exact same values for the combinators it shows:

Duplicate export. Try removing all but one export of the library. dart(duplicate_export)

[bwilkerson]

This sounds to me like a style preference. There's nothing semantically wrong with having multiple exports of different parts of the same library. (Though I agree that the more concise form is closer to the Dart style.)

That makes it a lint rather than a warning, but we're essentially not adding any new lints at this time that aren't requested by the language team, typically in order to facilitate adoption of new language features. So I suspect that we won't want to add this one.

[FMorschel]

I guess this is somewhat the same as what was asked at #59907 from your comment on #56830, but for exports.

[bwilkerson]

Yes, exports with hides have the same issue.

[FMorschel]

I've decided to leave #59907 as a request for handling the hide combinator warning.

Here I'm asking for an actual lint for merging equal imports/exports that differ show names.

---

@bwilkerson, do you think that if I have one import/export with a show combinator and another without any combinators at all like:

import 'b.dart';
import 'b.dart' show B; //does nothing since everything is shown above

We should add the warning here as a lint or at the current warnings for duplicate exports/imports? If you think it should be handled at the current warnings I can create a new issue for tracking that.

---

Edit

Just to point another similarly related issue: #56879