dart2js not compiling lambda with the correct parameter type.

[gmpassos]

I've encountered an unusual issue with dart2js and have isolated a code snippet that reproduces the bug:

• The code compiles successfully in both DDC and dart2js.
• It runs as expected in DDC.
• However, it fails to run when compiled with dart2js (in webdev release mode).

import 'dart:html';

void main() async {
  // Trigger voice loading...
  window.speechSynthesis?.getVoices();
  
  print('Loading voices...');
  await Future.delayed(Duration(seconds: 3));

  var voices = window.speechSynthesis?.getVoices();

  // Expects a non-empty list:
  print(voices);
  
  // Fails to call the `map` lambda when compiled by `dart2js`:
  var langs = voices?.map((e) => e.lang).toList();
  
  print(langs);  
}

• Note that the list voices needs to be populated to reproduce the bug.

• I'm not sure of the exact issue, but it seems that the compiled JS is expecting a different type for the map lambda parameter.

---

Run it:

 webdev serve -r

---

Open in the browser: http://127.0.0.1:8080

Browser Console output:

main.dart.js:3782 Loading voices...
main.dart.js:3782 [[object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice], [object SpeechSynthesisVoice]]
main.dart.js:535 Uncaught Error: TypeError: Closure 'main_closure': type '(SpeechSynthesisVoice) => String?' is not a subtype of type '(dynamic) => String?'
    at Object.wrapException (main.dart.js:535:43)
    at Object._failedAsCheck (main.dart.js:1581:15)
    at Rti._generalAsCheckImplementation [as _as] (main.dart.js:1570:9)
    at Rti._installSpecializedAsCheck (main.dart.js:1517:22)
    at JavaScriptObject.map$1$1 (main.dart.js:3860:80)
    at Object.map$1$1$ax (main.dart.js:371:44)
    at main.dart.js:3764:24
    at _wrapJsFunctionForAsync_closure.$protected (main.dart.js:2844:15)
    at _wrapJsFunctionForAsync_closure.call$2 (main.dart.js:4358:12)
    at _awaitOnObject_closure.call$1 (main.dart.js:4346:32)
wrapException @ main.dart.js:535
Error__throw @ main.dart.js:3282
Error_throwWithStackTrace @ main.dart.js:3330
call$0 @ main.dart.js:4727
_microtaskLoop @ main.dart.js:3038
_startMicrotaskLoop @ main.dart.js:3044
call$1 @ main.dart.js:4303
childList
call$1 @ main.dart.js:4313
_scheduleAsyncCallback @ main.dart.js:3058
_schedulePriorityAsyncCallback @ main.dart.js:3066
_rootHandleError @ main.dart.js:3103
_Future__propagateToListeners @ main.dart.js:2932
_completeError$2 @ main.dart.js:4548
_asyncRethrow @ main.dart.js:2817
(anonymous) @ main.dart.js:3745
(anonymous) @ main.dart.js:2844
call$2 @ main.dart.js:4358
call$1 @ main.dart.js:4346
runUnary$2$2 @ main.dart.js:4760
call$0 @ main.dart.js:4687
_Future__propagateToListeners @ main.dart.js:2975
_complete$1 @ main.dart.js:4532
call$0 @ main.dart.js:4374
call$0 @ main.dart.js:4339
setTimeout
_TimerImpl$2 @ main.dart.js:4332
_TimerImpl$ @ main.dart.js:2782
Timer__createTimer @ main.dart.js:2778
Timer_Timer @ main.dart.js:3099
Future_Future$delayed @ main.dart.js:2874
(anonymous) @ main.dart.js:3755
(anonymous) @ main.dart.js:2844
call$2 @ main.dart.js:4358
_asyncStartSync @ main.dart.js:2789
main @ main.dart.js:3772
(anonymous) @ main.dart.js:6201
(anonymous) @ main.dart.js:6182
dartProgram @ main.dart.js:6195
(anonymous) @ main.dart.js:6204Understand this error

---

$> dart info

If providing this information as part of reporting a bug, please review the information
below to ensure it only contains things you're comfortable posting publicly.

#### General info

- Dart 3.5.3 (stable) (Wed Sep 11 16:22:47 2024 +0000) on "macos_x64"
- on macos / Version 12.7.4 (Build 21H1123)
- locale is en-BR

#### Project info

- sdk constraint: '^3.5.2'
- dependencies: web
- dev_dependencies: build_runner, build_web_compilers, lints

#### Process info

No Dart processes found.

---

pubspec.lock:

# Generated by pub
# See https://dart.dev/tools/pub/glossary#lockfile
packages:
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      url: "https://pub.dev"
    source: hosted
    version: "73.0.0"
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    description: dart
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    description:
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      url: "https://pub.dev"
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    description:
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    description:
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    description:
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    description:
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      url: "https://pub.dev"
    source: hosted
    version: "1.4.2"
  shelf_web_socket:
    dependency: transitive
    description:
      name: shelf_web_socket
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      url: "https://pub.dev"
    source: hosted
    version: "2.0.0"
  source_maps:
    dependency: transitive
    description:
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      url: "https://pub.dev"
    source: hosted
    version: "0.10.12"
  source_span:
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    description:
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      url: "https://pub.dev"
    source: hosted
    version: "1.10.0"
  stack_trace:
    dependency: transitive
    description:
      name: stack_trace
      sha256: "9f47fd3630d76be3ab26f0ee06d213679aa425996925ff3feffdec504931c377"
      url: "https://pub.dev"
    source: hosted
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    dependency: transitive
    description:
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      sha256: ba2aa5d8cc609d96bbb2899c28934f9e1af5cddbd60a827822ea467161eb54e7
      url: "https://pub.dev"
    source: hosted
    version: "2.1.2"
  stream_transform:
    dependency: transitive
    description:
      name: stream_transform
      sha256: "14a00e794c7c11aa145a170587321aedce29769c08d7f58b1d141da75e3b1c6f"
      url: "https://pub.dev"
    source: hosted
    version: "2.1.0"
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    description:
      name: string_scanner
      sha256: "688af5ed3402a4bde5b3a6c15fd768dbf2621a614950b17f04626c431ab3c4c3"
      url: "https://pub.dev"
    source: hosted
    version: "1.3.0"
  term_glyph:
    dependency: transitive
    description:
      name: term_glyph
      sha256: a29248a84fbb7c79282b40b8c72a1209db169a2e0542bce341da992fe1bc7e84
      url: "https://pub.dev"
    source: hosted
    version: "1.2.1"
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    dependency: transitive
    description:
      name: test_api
      sha256: "664d3a9a64782fcdeb83ce9c6b39e78fd2971d4e37827b9b06c3aa1edc5e760c"
      url: "https://pub.dev"
    source: hosted
    version: "0.7.3"
  timing:
    dependency: transitive
    description:
      name: timing
      sha256: "70a3b636575d4163c477e6de42f247a23b315ae20e86442bebe32d3cabf61c32"
      url: "https://pub.dev"
    source: hosted
    version: "1.0.1"
  typed_data:
    dependency: transitive
    description:
      name: typed_data
      sha256: facc8d6582f16042dd49f2463ff1bd6e2c9ef9f3d5da3d9b087e244a7b564b3c
      url: "https://pub.dev"
    source: hosted
    version: "1.3.2"
  watcher:
    dependency: transitive
    description:
      name: watcher
      sha256: "3d2ad6751b3c16cf07c7fca317a1413b3f26530319181b37e3b9039b84fc01d8"
      url: "https://pub.dev"
    source: hosted
    version: "1.1.0"
  web:
    dependency: "direct main"
    description:
      name: web
      sha256: "97da13628db363c635202ad97068d47c5b8aa555808e7a9411963c533b449b27"
      url: "https://pub.dev"
    source: hosted
    version: "0.5.1"
  web_socket:
    dependency: transitive
    description:
      name: web_socket
      sha256: "3c12d96c0c9a4eec095246debcea7b86c0324f22df69893d538fcc6f1b8cce83"
      url: "https://pub.dev"
    source: hosted
    version: "0.1.6"
  web_socket_channel:
    dependency: transitive
    description:
      name: web_socket_channel
      sha256: "9f187088ed104edd8662ca07af4b124465893caf063ba29758f97af57e61da8f"
      url: "https://pub.dev"
    source: hosted
    version: "3.0.1"
  yaml:
    dependency: transitive
    description:
      name: yaml
      sha256: "75769501ea3489fca56601ff33454fe45507ea3bfb014161abc3b43ae25989d5"
      url: "https://pub.dev"
    source: hosted
    version: "3.1.2"
sdks:
  dart: ">=3.5.2 <3.7.0"

[dart-github-bot]

Summary: The issue is that dart2js fails to compile a lambda function used with the map method on a list of SpeechSynthesisVoice objects, resulting in a type error. The code compiles and runs correctly in DDC, but fails in dart2js webdev release mode.

[gmpassos]

If you need to investigate the generated JS code, force dart2js to not optimize and minify it:

build.yaml:

targets:
  $default:
    builders:
      build_web_compilers:entrypoint:
        options:
          dart2js_args:
            - -O1
            - --no-minify
            - --disable-program-split
            - --no-source-maps

[fishythefish]

The inferred static type of the closure is indeed (SpeechSynthesisVoice) => String?. This is expected, since voices has static type List<SpeechSynthesisVoice>?. However, at runtime, print(voices.runtimeType) shows that it's actually JSArray<dynamic>, so .map expects a closure whose parameter type is dynamic, hence the failure.

This is because the array ultimately comes directly from window.speechSynthesis.getVoices() in JS, which doesn't tag the result with the element type.

/cc @srujzs @rakudrama

[srujzs]

I guess the real issue then is that List<SpeechSynthesisVoice> is not sound here with respect to the generic type, but that's a known issue in how we intercept JSArray. The workaround then would just be to use a lambda that accepts dynamic, but that seems ugly.

Consider using package:web instead where you'll get better static typing: https://github.com/dart-lang/web/blob/8478cd27d574249eca3d41f9135458dfda2762c8/web/lib/src/dom/speech_api.dart#L319.

[gmpassos]

I guess the real issue then is that List<SpeechSynthesisVoice> is not sound here with respect to the generic type, but that's a known issue in how we intercept JSArray. The workaround then would just be to use a lambda that accepts dynamic, but that seems ugly.

Consider using package:web instead where you'll get better static typing: https://github.com/dart-lang/web/blob/8478cd27d574249eca3d41f9135458dfda2762c8/web/lib/src/dom/speech_api.dart#L319.

For me this is a bug on dart2js, specially because DDC works. This shouldn't be treated as a specific bug on dart:html.

[srujzs]

For me this is a bug on dart2js, specially because DDC works. This shouldn't be treated as a specific bug on dart:html.

I agree (although DDC working doesn't necessarily indicate that), but this is a problem that materializes specifically for dart:html and older interop versions. There are a few possible solutions e.g. adding the generic type manually, but I'm not sure what all the corner cases there are yet. We can keep this open, but it is preferable to use package:web to avoid these issues that may take much longer to resolve e.g.

import 'package:web/web.dart' as web;

void main() {
  web.window.speechSynthesis.getVoices().toDart.map((e) => e.lang).toList();
}