dart2js is generating a phantom type and throwing bogus errors about it

[nex3]

I'm seeing a very bizarre issue where dart2js is generating two distinct JS classes for a single Dart class, and throwing type errors because an object of one of these classes isn't an instance of the other. I haven't been able to find a minimal reproduction for this, so unfortunately you'll need to check out Dart Sass as a whole:

$ git clone git://github.com/sass/dart-sass --branch dart2js-type-error-repro
$ cd dart-sass
$ pub get
$ pub run grinder pkg-npm-dev
$ node repro.js

This will produce the following error:

result is SassNull: true
{
  err: Error: TypeError: Instance of 'SassNull': type 'SassNull' is not a subtype of type 'FutureOr<SassNull0>?'
      at Object._newRenderError (/home/nweiz/goog/sass/dart/build/npm/sass.dart.js:13677:19)
      at _render_closure1.call$2 (/home/nweiz/goog/sass/dart/build/npm/sass.dart.js:84642:21)
      at _RootZone.runBinary$3$3 (/home/nweiz/goog/sass/dart/build/npm/sass.dart.js:28567:18)
      at _FutureListener.handleError$1 (/home/nweiz/goog/sass/dart/build/npm/sass.dart.js:26968:26)
      at _Future__propagateToListeners_handleError.call$0 (/home/nweiz/goog/sass/dart/build/npm/sass.dart.js:27288:49)
      at Object._Future__propagateToListeners (/home/nweiz/goog/sass/dart/build/npm/sass.dart.js:4638:77)
      at _Future._completeError$2 (/home/nweiz/goog/sass/dart/build/npm/sass.dart.js:27111:9)
      at _AsyncAwaitCompleter.completeError$2 (/home/nweiz/goog/sass/dart/build/npm/sass.dart.js:26733:12)
      at Object._asyncRethrow (/home/nweiz/goog/sass/dart/build/npm/sass.dart.js:4436:17)
      at /home/nweiz/goog/sass/dart/build/npm/sass.dart.js:13315:20 {
    formatted: "Error: TypeError: Instance of 'SassNull': type 'SassNull' is not a subtype of type 'FutureOr<SassNull0>?'",
    status: 3
  },
  result: null
}

This error is being thrown by this code. Note that:

• dart2js thinks there are two distinct types: SassNull and SassNull0.
• result is SassNull returns true, despite the fact that SassNull in that context should refer to the same type as the return type declaration.
• This only happens for the SassNull type. There are various other names (such as SassNumber and SassString) that actually do apply to multiple types, but they don't exhibit this behavior since one of the types extends the other.
• This only happens in Sass's asynchronous compilation mode, likely because the type check is being performed by the completer generated by the async/await transformation.

This is a fairly severe issue for us, since it means that we essentially can't run asynchronous JS compilations in development mode which makes it difficult to catch real type errors.

[sigmundch]

/cc @fishythefish @rakudrama

[rakudrama]

I was looking at occurences of SassNull and SassNull0 and I noticed that this closure from meta.dart appears one in the source, but twice in the compiled output:

  _function("type-of", r"$value", (arguments) {
    var value = arguments[0];
    if (value is SassArgumentList) {
      return SassString("arglist", quotes: false);
    }
    if (value is SassBoolean) return SassString("bool", quotes: false);
    if (value is SassColor) return SassString("color", quotes: false);
    if (value is SassList) return SassString("list", quotes: false);
    if (value is SassMap) return SassString("map", quotes: false);
    if (value is SassNull) return SassString("null", quotes: false);
    if (value is SassNumber) return SassString("number", quotes: false);
    if (value is SassFunction) return SassString("function", quotes: false);
    assert(value is SassString);
    return SassString("string", quotes: false);
  }),

one occurrence has

      if (value instanceof A.SassMap)
        return new D.SassString("map", false);
      if (value instanceof O.SassNull)
        return new D.SassString("null", false);
      if (value instanceof T.SassNumber)

the other:

      if (value instanceof A.SassMap0)
        return new D.SassString0("map", false);
      if (value instanceof O.SassNull0)
        return new D.SassString0("null", false);
      if (value instanceof T.SassNumber0)

How is it that one closure appears twice in the output?
Could it be that some files are copied and imported from a different URI?

I also see the example code twice in an async function and twice in a regular function, for a total of four times:

./build/sass.dart.js\053971-              // Function start
./build/sass.dart.js\053972:              P.print("result is SassNull: true");
./build/sass.dart.js\053973-              $async$returnValue = C.C_SassNull0;
--
./build/sass.dart.js\059891-    visitNullExpression$1: function(node) {
./build/sass.dart.js\059892:      P.print("result is SassNull: true");
./build/sass.dart.js\059893-      return C.C_SassNull0;
--
./build/sass.dart.js\069042-              // Function start
./build/sass.dart.js\069043:              P.print("result is SassNull: true");
./build/sass.dart.js\069044-              $async$returnValue = C.C_SassNull;
--
./build/sass.dart.js\077697-    visitNullExpression$1: function(node) {
./build/sass.dart.js\077698:      P.print("result is SassNull: true");
./build/sass.dart.js\077699-      return C.C_SassNull;

I put a print in dart2js and the two SassNull classes have different canonical URIs.
This means that they are different classes in different libraries.

  Class j:class(SassNull)  package:sass/src/value/null.dart
  Class j:class(SassNull)  file:///usr/local/google/home/sra/dart-sass/lib/src/value/null.dart

As far as dart2js is concerned, anything to do with URIs happens in the common front end.

/cc @johnniwinther Do you have any ideas what might be happening here?

[johnniwinther]

The problem is that from the entrypoint, package:sass is imported through both a file URI and a package URI.

The (generated?) entrypoint has the lines

import "../bin/sass.dart" as cli_pkg_main_0_;
import "../lib/src/node.dart" as module_main; 

where the former imports package:sass through a package import and the later is imported relative to the entry-point, which is a file URI. This means that Dart sees these as different sets of libraries, and therefore having to distinct types name Sass.

If the entrypoint is changed to import src/node.dart through a package import, this should fix the problem:

import "../bin/sass.dart" as cli_pkg_main_0_;
import "package:sass/src/node.dart" as module_main;

[nex3]

That does seem like something we should fix, but I don't think it explains the problem here. None of the members of cli_pkg_main_0_ are actually accessed in the example in question. What's more, the name SassNull in _EvaluateVisitor.visitNullExpression should refer to the same class in every context, but result is SassNull is true whereas the implicit type-check that return result is-a FutureOr<SassNull> fails.