[dart2wasm] Null assertion in code generator on using extension type type parameter in success function in a Promise

[srujzs]

Dart SDK version: Dart SDK version: 3.3.0-edge.2960e60e24056266f3661bfdb11163a98b3050b1 (main) (Wed Nov 29 13:13:43 2023 -0800) on "macos_x64"
Browser: d8

// SharedOptions=--enable-experiment=inline-class

import 'dart:async';
import 'dart:js_interop';

extension type MyJSPromise<T extends JSAny?>(JSObject s) {
  external void then(JSFunction success, JSFunction error);

  Future<T> get toDart {
    final completer = Completer<T>();
    final success = (T r) {
      completer.complete(r);
    }.toJS;
    final error = (JSAny? e) {}.toJS;
    then(success, error);
    return completer.future;
  }
}

void main() {}

outputs:

stdout:
Exception in TypeLiteral at null

stderr:
Unhandled exception:
Null check operator used on a null value
#0      CodeGenerator.instantiateTypeParameter (package:dart2wasm/code_generator.dart:3052)
#1      Types.makeType (package:dart2wasm/types.dart:572)
#2      CodeGenerator.visitTypeLiteral (package:dart2wasm/code_generator.dart:2985)
#3      TypeLiteral.accept1 (package:kernel/ast.dart:7990)
#4      CodeGenerator.wrap (package:dart2wasm/code_generator.dart:878)
#5      CodeGenerator.makeListFromExpressions.<anonymous closure> (package:dart2wasm/code_generator.dart:2927)
#6      Translator.makeList (package:dart2wasm/translator.dart:1004)
#7      CodeGenerator.makeList (package:dart2wasm/code_generator.dart:2918)
#8      CodeGenerator.makeListFromExpressions (package:dart2wasm/code_generator.dart:2926)
#9      Types._makeTypeList (package:dart2wasm/types.dart:411)
#10     Types._makeFunctionType (package:dart2wasm/types.dart:514)
#11     Types.makeType (package:dart2wasm/types.dart:595)
#12     Types.emitTypeCheck (package:dart2wasm/types.dart:658)
#13     CodeGenerator.visitAsExpression (package:dart2wasm/code_generator.dart:3008)
#14     AsExpression.accept1 (package:kernel/ast.dart:7643)
#15     CodeGenerator.wrap (package:dart2wasm/code_generator.dart:878)
#16     CodeGenerator.visitFunctionInvocation (package:dart2wasm/code_generator.dart:2622)
#17     FunctionInvocation.accept1 (package:kernel/ast.dart:6071)
#18     CodeGenerator.wrap (package:dart2wasm/code_generator.dart:878)
#19     CodeGenerator.visitArgumentsLists (package:dart2wasm/code_generator.dart:2770)
#20     CodeGenerator._visitArguments (package:dart2wasm/code_generator.dart:2804)
#21     CodeGenerator.visitStaticInvocation (package:dart2wasm/code_generator.dart:1755)
#22     StaticInvocation.accept1 (package:kernel/ast.dart:6584)
#23     CodeGenerator.wrap (package:dart2wasm/code_generator.dart:878)
#24     CodeGenerator.visitReturnStatement (package:dart2wasm/code_generator.dart:1523)
#25     ReturnStatement.accept (package:kernel/ast.dart:10163)
#26     CodeGenerator.visitStatement (package:dart2wasm/code_generator.dart:889)
#27     CodeGenerator.visitBlock (package:dart2wasm/code_generator.dart:1011)
#28     Block.accept (package:kernel/ast.dart:9199)
#29     CodeGenerator.visitStatement (package:dart2wasm/code_generator.dart:889)
#30     CodeGenerator.generateBody (package:dart2wasm/code_generator.dart:578)
#31     CodeGenerator.generate (package:dart2wasm/code_generator.dart:234)
#32     Translator.translate (package:dart2wasm/translator.dart:349)
#33     compileToModule (package:dart2wasm/compile.dart:151)
<asynchronous suspension>
#34     main (package:dart2wasm/dart2wasm.dart:144)
<asynchronous suspension>
#35     main (file:///Users/srujzs/dart-sdk/sdk/pkg/dart2wasm/bin/dart2wasm.dart:7)
<asynchronous suspension>

I tried to workaround this by using a Completer<JSAny?> in a few different ways but the cast to T later fails e.g. undefined:0: [object WebAssembly.Exception] or RuntimeError: illegal cast, so I'm guessing that might be a consequence of the codegen not handling this type parameter correctly.

[sigmundch]

Mmm, it appears this happens because of the .toJS call to convert the closure. This program also produces a similar crash:

import 'dart:js_interop';

void m<T>() {
  final f = (T x) {};
  f.toJS;
}

void main() { }

Note that if we use a static tearoff instead of a closure, we'd get a static error because we don't allow calling .toJS on a function that takes generics:

import 'dart:js_interop';
void m3<T extends JSAny?>(T t) {}
void main() { m3.toJS; }

This produces the error:

example.dart:3:18: Error: Type 'T' is not a valid type for external `dart:js_interop` APIs. The only valid types are: @staticInterop types, JS types from `dart:js_interop`, void, bool, num, double, int, String, and any extension type that erases to one of these types.
Use one of the valid types instead.

[srujzs]

I think you discovered two others bugs in our error checker that needs to be fixed :)

1. We should be checking local functions for type parameter types. I believe the error checker assumes we don't need to do this because it's checked elsewhere, but that's only for top-level procedures, not locals. Note that the first snippet's T is not bound to anything, so there should be an error.
2. That second code snippet should be invalid only because it declares type parameters. T is bound to a static interop type whose subtypes' can only be other static interop types, so the usage of that T shouldn't be invalid.

That being said, it does look like .toJS is the culprit there, so I'll need to debug further.

[srujzs]

Oh, I'm silly. I already filed a similar bug on this: #53968.

[osa1]

Since this isn't a dart2wasm bug, what would be the right label for it?

[srujzs]

Let's label this as a JS interop bug for now (but only affects dart2wasm, so I'll keep that label too).