Dart 2 frontend uses the wrong scope for type annotations

[rmacnak-google]

This is a simplification of an issue encountered with generated FIDL2 bindings in Fuchsia. It blocks enabling Dart 2 in Fuchsia.

import 'dart:math' as math;

class Foo {
  List<math.Point> math;
}

main() {
  print(new Foo());
}

works in Dart, but in Dart 2 yields

a.dart:4:8: Error: 'math.Point' can't be used as a type because 'math' doesn't refer to an import prefix.
  List<math.Point> math;
       ^^^^^^^^^^

@peter-ahe-google @a-siva @zanderso

[eernstg]

The behavior that you've seen in Dart 2 is correct, and the behavior that you've seen elsewhere is a bug.

Dart does not have a notion of putting declared names into different namespaces according to the nature of their declaration (with one exception: labels). So math in the type annotation is looked up in the current scope, which is the instance scope of Foo, and in that scope the result will be the instance variable. It is a bug for any Dart tool (Dart 1 as well as 2) to resolve that occurrence of math to the library prefix.

Taking that into account, wouldn't it be possible to generate code which doesn't have that name clash?

[kmillikin]

For what it's worth, this is a duplicate of #32200

[eernstg]

Also note #32248 which requests the relevant bug fix in the analyzer.

[zanderso]

@eernstg Despite possibly being a bug, this is long-standing behavior of the VM and apparently also the analyzer, so I think we should consider the new behavior to be a breaking change. Maybe we should have a Dart 2 breaking change announcement from the language team for this. /cc @leafpetersen @a-siva

@rmacnak-google Do you think that mangling the generated library prefixes with a leading '$' would be sufficient? Can library prefixes have a leading '$'? /cc @abarth.

[eernstg]

Funny, the analyzer isn't even quite sure whether it's looking up math as a prefix:

import 'dart:math' as math;

class Foo {
  List<math.Point> math;
}

main() => print(new Foo());

produces

  hint • Unused import: 'dart:math' at n007.dart:4:8 • unused_import

I understand that even a bug fix can be a breaking change, organizationally. I hope it's manageable! On the other hand, I think we should maintain the property that "Dart has one name space" as far as possible (and nobody knows about labels, anyway ;-).

[leafpetersen]

Posting an announcement about this seems like a reasonable idea, I can do.

[vsmenon]

Is this still a P0?

[vsmenon]

Dropping the P0 as it appears @rmacnak-google unblocked Fuchsia. Please bump if I misunderstood.

Is the only open here to communicate the change?

[leafpetersen]

I've announced that this is de facto changing. We have a bug (#32248) for the analyzer to fix this, everything else gets this from the CFE. Closing this, but re-open if there are any remaining issues that I've missed.