dart2wasm: Failed null check at ClosureRepresentation.fieldIndexForSignature on a statically typed constructor tearoff expression

[plammens]

I get a compilation error when compiling to wasm with flutter build web --wasm --release (that I don't get with other compilation targets.) (I use a flutter command but the error happens in the dart2wasm library, so I'm posting the issue here.)

The error is:

Target dart2wasm failed: ProcessException: Process exited abnormally with exit code 64:
*NOTE*: Compilation to WasmGC is experimental.
The support may change, or be removed, with no advance notice.

Exception in ConstantExpression at [...]:70:21

Unhandled exception:
Null check operator used on a null value

Stack trace:

#0      ClosureRepresentation.fieldIndexForSignature (package:dart2wasm/closures.dart:112)
#1      ConstantCreator.visitInstantiationConstant.<anonymous closure>.fillVtableEntry (package:dart2wasm/constants.dart:725)
#2      ConstantCreator.visitInstantiationConstant.<anonymous closure>.makeVtable (package:dart2wasm/constants.dart:749)
#3      ConstantCreator.visitInstantiationConstant.<anonymous closure> (package:dart2wasm/constants.dart:765)
#4      ConstantCreator.createConstant (package:dart2wasm/constants.dart:285)
#5      ConstantCreator.visitInstantiationConstant (package:dart2wasm/constants.dart:698)
#6      InstantiationConstant.accept (package:kernel/ast.dart:14209)
#7      ConstantCreator.ensureConstant (package:dart2wasm/constants.dart:252)
#8      ConstantInstantiator.defaultConstant (package:dart2wasm/constants.dart:137)
#9      ConstantVisitorDefaultMixin.visitStaticTearOffConstant (package:kernel/visitor.dart:944)
#10     InstantiationConstant.accept (package:kernel/ast.dart:14209)
#11     ConstantInstantiator.instantiate (package:dart2wasm/constants.dart:121)
#12     Constants.instantiateConstant (package:dart2wasm/constants.dart:103)
#13     CodeGenerator.visitConstantExpression (package:dart2wasm/code_generator.dart:2841)
#14     ConstantExpression.accept1 (package:kernel/ast.dart:8707)
#15     CodeGenerator.wrap (package:dart2wasm/code_generator.dart:852)
#16     CodeGenerator.visitRecordLiteral (package:dart2wasm/code_generator.dart:3086)
#17     RecordLiteral.accept1 (package:kernel/ast.dart:8490)
#18     CodeGenerator.wrap (package:dart2wasm/code_generator.dart:852)
#19     CodeGenerator.visitLet (package:dart2wasm/code_generator.dart:1669)
#20     Let.accept1 (package:kernel/ast.dart:8775)
#21     CodeGenerator.wrap (package:dart2wasm/code_generator.dart:852)
#22     CodeGenerator.visitLet (package:dart2wasm/code_generator.dart:1669)
#23     Let.accept1 (package:kernel/ast.dart:8775)
#24     CodeGenerator.wrap (package:dart2wasm/code_generator.dart:852)
#25     CodeGenerator.visitMapLiteral.<anonymous closure> (package:dart2wasm/code_generator.dart:2956)
#26     Translator.makeArray (package:dart2wasm/translator.dart:1025)
#27     CodeGenerator.makeArray (package:dart2wasm/code_generator.dart:2921)
#28     CodeGenerator.visitMapLiteral (package:dart2wasm/code_generator.dart:2949)
#29     MapLiteral.accept1 (package:kernel/ast.dart:8341)
#30     CodeGenerator.wrap (package:dart2wasm/code_generator.dart:852)
#31     CodeGenerator.visitVariableDeclaration (package:dart2wasm/code_generator.dart:1020)
#32     VariableDeclaration.accept (package:kernel/ast.dart:10791)
#33     CodeGenerator.visitStatement (package:dart2wasm/code_generator.dart:863)
#34     CodeGenerator.visitBlock (package:dart2wasm/code_generator.dart:983)
#35     Block.accept (package:kernel/ast.dart:9247)
#36     CodeGenerator.visitStatement (package:dart2wasm/code_generator.dart:863)
#37     CodeGenerator.generateBody (package:dart2wasm/code_generator.dart:557)
#38     CodeGenerator.generate (package:dart2wasm/code_generator.dart:236)
#39     Translator.translate (package:dart2wasm/translator.dart:374)
#40     compileToModule (package:dart2wasm/compile.dart:188)
<asynchronous suspension>
#41     generateWasm (package:dart2wasm/generate_wasm.dart:24)
<asynchronous suspension>
#42     main (file:///C:/b/s/w/ir/x/w/sdk/pkg/dart2wasm/bin/dart2wasm.dart:10)
<asynchronous suspension>

I've tried to write a minimal reproducible example from scratch, but I haven't succeeded, so I'll show my actual code.

The culprit of this exception, as pointed out by the error, seems to be this tearoff expression:

final Map<Type, _TypeConfig> typesConfig = {
      // [...]
      Habit: (
        enabled: isSubtype<Habit, C>(),
        typeInfo: CommitmentTypeInfo(
          name: "Habit",
          icon: Habit.icon_,
          orderKey: DiscreteHabit.commitmentTypeInfo.orderKey,
          fromMapFactory: (map) => throw UnimplementedError(),
        ),
        editScreen: EditHabit.blank   // ERROR: this is the constant expression that causes the error
      ),
     // [...]
    };

I have the following typedefs:

typedef _EditPageBuilder<C extends Commitment> = Widget Function({
  CompositeCommitment<C>? parentCommitment,
  Function(C)? onEdited,
});

typedef _TypeConfig<C extends Commitment> = ({
  bool enabled,
  CommitmentTypeInfo typeInfo,
  _EditPageBuilder<C> editScreen,
});

The problem disappears when I set:

typedef _EditPageBuilder<C extends Commitment> = dynamic;

Related code (definition of EditHabit.blank):

class EditHabit<H extends Habit> extends EditCommitment<H> {
  // [...]

  EditHabit.blank({
    super.key,
    super.parentCommitment,
    super.onEdited,
  }) : super.blank(
          commitmentTypeInfo: DiscreteHabit.commitmentTypeInfo,
          titleHintText: titleHint,
        );

  // [...]
}

abstract class EditCommitment<C extends Commitment> extends Edit<C> {
  // [...]

  /// For creating a new commitment.
  const EditCommitment.blank({
    super.key,
    required CommitmentTypeInfo commitmentTypeInfo,
    required super.titleHintText,
    this.parentCommitment,
    super.onEdited,
  })  : _commitmentTypeInfo = commitmentTypeInfo,
        super.blank();

  // Should be CompositeCommitment<C> but Dart forces the type parameter to be
  // covariant, which is not desirable here.
  final CompositeCommitment? parentCommitment;

  // [...]
}

abstract class Edit<O extends CommitmentStoreObjectSnapshot>
    extends ConsumerStatefulWidget {
  // [...]

  const Edit.blank({
    super.key,
    required this.titleHintText,
    this.onEdited,
  }) : object = null;

  /// Called after the edit/creation has been successfully executed.
  final Function(O)? onEdited;

  // [...]
}

Surprisingly, I have other elements in the map above for which this error does not occur, in particular for EditTask.blank:

class EditTask extends EditCommitment<Task> {
  // [...]

  const EditTask.blank({
    super.parentCommitment,
    super.key,
    super.onEdited,
  }) : super.blank(
          commitmentTypeInfo: Task.commitmentTypeInfo,
          titleHintText: titleHint,
        );

  // [...]
}

I thought the difference might be in the const, but I haven't managed to make a minimal reproducible example. Maybe someone experienced with dart2wasm will be able to do so.

---

• Dart version and tooling diagnostic info (dart info)

#### General info

- Dart 3.4.4 (stable) (Wed Jun 12 15:54:31 2024 +0000) on "windows_x64"
- on windows / "Windows 10 Home" 10.0 (Build 22631)
- locale is en-GB

#### Project info

- sdk constraint: '^3.4.0'
- dependencies: built_collection, cloud_firestore, collection, cupertino_icons, easy_debounce, equatable, firebase_auth, firebase_core, firebase_ui_auth, firebase_ui_oauth_google, flut
ter, flutter_fancy_tree_view, flutter_local_notifications, flutter_localizations, flutter_riverpod, flutter_speed_dial, flutter_timezone, google_sign_in, intersperse, intl, meta, modal_bottom_sheet, package_info_plus, quiver, rrule_generator, skeletonizer, sliver_tools, stream_transform, timezone, url_launcher
- dev_dependencies: custom_lint, flutter_launcher_icons, flutter_lints, flutter_test, riverpod_lint, test
- elided dependencies: 2

#### Process info

| Memory | CPU | Elapsed time | Command line |
| -----: | --: | -----------: | ------------ |
| 274 MB |  -- |              | dart.exe     |
|  30 MB |  -- |              | dart.exe     |
|   3 MB |  -- |              | dart.exe     |
| 476 MB |  -- |              | dart.exe     |
| 485 MB |  -- |              | dart.exe     |
|  94 MB |  -- |              | dart.exe     |
| 100 MB |  -- |              | dart.exe     |

• Whether you are using Windows, macOS, or Linux (if applicable)
  Windows

• Whether you are using Chrome, Safari, Firefox, Edge (if applicable)
  Chrome

[dart-github-bot]

Summary: The dart2wasm compiler fails to compile a statically typed constructor tearoff expression used in a Map literal, resulting in a null check error during constant evaluation. The issue occurs when compiling to WasmGC and is not observed with other compilation targets.

[mkustermann]

/cc @osa1 Could you have a look?

[osa1]

I've tried to write a minimal reproducible example from scratch, but I haven't succeeded, so I'll show my actual code.

I'm unable to reproduce the issue from the snippets. Is it possible to share the whole project with commands to run to get the error?

[plammens]

I'm unable to reproduce the issue from the snippets. Is it possible to share the whole project with commands to run to get the error?

It's a private repo, maybe I can add you as a collaborator temporarily so you can access it?

[osa1]

@plammens unfortunately we are only allowed to look at appropriately licensed open source code.

(Not sure which licenses are those exactly, but common ones like BSD, MIT etc. work)

For now I will try a few more things to try to reproduce this.

[mkustermann]

@plammens Could you try to verify that this problem still happens when you're on flutter master channel?

The problem disappears when I set:

typedef _EditPageBuilder<C extends Commitment> = dynamic;

This is an indication that the issue is due to a partial instantiation of a tear-off constant, i.e. EditHabit is generic and will get its type argument filled in when EditHabit.blank is used in a context where one doesn't expect a generic function but one without type arguments, the language type inference.

I thought the difference might be in the const, but I haven't managed to make a minimal reproducible example. Maybe someone experienced with dart2wasm will be able to do so

The issue has to do with closure representations. Dart2wasm will try to make a good calling convention for closures. To do that it will look at a) all function definitions b) all function call sites. It then partitions them together into groups that have to be together (all possible definitions for a given callsite, all callsites for a definition) and based on those groups makes optimized vtables for fast closure calls.

If this issue still persists on flutter master channel, then we'll need a minimal reproduction we can look at (I've spent the entire morning looking through the relevant code and trying to construct cases that could trigger this, but failed to come up with an example where this would trigger).

What we need for a minimal reproduction is the code that creates those closures (class hierarchy, constructors and how the constructor tear-offs are used) and the code that invokes those closures. @plammens Could you try to make a minimal flutter app that reproduces this?

[plammens]

@plammens Could you try to verify that this problem still happens when you're on flutter master channel?

I can confirm that the issue is still there on master, flutter/flutter@e66ce15 .

```text Flutter 3.24.0-1.0.pre.517 • channel master • https://github.com/flutter/flutter.git Framework • revision e66ce1591f (16 minutes ago) • 2024-08-08 14:25:19 -0400 Engine • revision 387f6f3c5f Tools • Dart 3.6.0 (build 3.6.0-129.0.dev) • DevTools 2.38.0 ```

What we need for a minimal reproduction is the code that creates those closures (class hierarchy, constructors and how the constructor tear-offs are used) and the code that invokes those closures. @plammens Could you try to make a minimal flutter app that reproduces this?

I'll try again making a reproducible example, this time I'll try to work my way down from my actual code to a more minimal version instead of trying to build my way up.

[plammens]

I've used a script to put all my code in a single file to try to whittle it down to a minimal example, but it seems this is going to take forever because many times I delete some code that seems completely unrelated and the error goes away. Plus I have to wait 60 seconds between each iteration due to having to run the command. At least it does seem to be deterministic with respect to the code.

Currently gone from 10k lines of code to 7.5k. Is there any chance I can just send you the file privately and slap an open-source license on it? It's a personal project anyway.

[mkustermann]

I've used a script to put all my code in a single file to try to whittle it down to a minimal example, but it seems this is going to take forever because many times I delete some code that seems completely unrelated and the error goes away. Plus I have to wait 60 seconds between each iteration due to having to run the command.

We really appreciate the effort, @plammens !

Currently gone from 10k lines of code to 7.5k. Is there any chance I can just send you the file privately and slap an open-source license on it? It's a personal project anyway.

If it's under an open source license we can take a look - irrespective of how it's shared. You're welcome to share via email.

[plammens]

If it's under an open source license we can take a look - irrespective of how it's shared. You're welcome to share via email.

Okay, I've added a GPL license to the file. What email address should I send it to? I'll also send the pubspec in case you need it.

By the way, don't try to make too much sense of the code as it is now in a heavily mutilated state :)

[mkustermann]

Okay, I've added a GPL license to the file. What email address should I send it to? I'll also send the pubspec in case you need it.

You can send it to [email protected]

[plammens]

You can send it to [email protected]

Done, thanks!

[mkustermann]

@plammens Thank you very much for the reproduction!

The following test case reproduces the issue

void main() {
  final funs = <String Function({Object? shared})>[
    foo,
    bar,
  ];

  final one = int.parse('1');
  final barS = funs[one] as String Function({Object? barSpecific});
  if (barS(barSpecific: 1) != 'bar(null, 1)') {
    throw 'failed: ${barS(barSpecific: 1)}';
  }
}

String foo<T>({Object? shared, Object? fooSpecific}) =>
    'foo<$T>($shared, $fooSpecific)';

String bar({Object? shared, Object? barSpecific}) =>
    'bar($shared, $barSpecific)';

=> Sent out cl/380100

[osa1]

Fixed with f7fc4d0.

[mkustermann]

@plammens The fix should have rolled up all the way to flutter/flutter now, so it's available on main/master channel