Erroneous RangeError when compiling to WASM

[Rexios80]

flyerhq/flutter_chat_ui#596

Unhandled exception:
RangeError (index): Invalid value: Valid value range is empty: 1
#0      List.[] (dart:core-patch/growable_array.dart:264)
#1      Intrinsifier.generateInstanceIntrinsic (package:dart2wasm/intrinsics.dart:337)
#2      CodeGenerator.visitInstanceInvocation (package:dart2wasm/code_generator.dart:1817)
#3      InstanceInvocation.accept1 (package:kernel/ast.dart:5803)

String _multiUserTextBuilder(List<types.User> author) {
  if (author.isEmpty) {
    return '';
  } else if (author.length == 1) {
    return '${author.first.firstName} is typing';
  } else if (author.length == 2) {
    return '${author.first.firstName} and ${author[1].firstName}'; // This line is the issue
  } else {
    return '${author.first.firstName} and ${author.length - 1} others';
  }
}

As far as I can tell, this is perfectly valid Dart code failing to compile to WASM

See also: flutter/flutter#148617

[kevmoo]

@Rexios80 – confirm 3.5.0 (build 3.5.0-167.0.dev) – right?

[osa1]

I've started debugging this, but the package cannot be built right now without making some changes in the dependencies, see flyerhq/flutter_chat_ui#596 (comment).

[Rexios80]

@kevmoo

% flutter --version
Flutter 3.22.0-43.0.pre.18 • channel [user-branch] • unknown source
Framework • revision 7b51c0c336 (10 minutes ago) • 2024-05-23 08:55:23 -0400
Engine • revision 8b094fbb94
Tools • Dart 3.5.0 (build 3.5.0-178.0.dev) • DevTools 2.36.0-dev.10

[mkustermann]

I think this is an incorrect optimization: We see that the List<types.User> author list is actually always a const <types.User>[]. Then our compiler wants to optimize the code by performing the list index at compile-time, but it doesn't check whether the index is in-bounds.

=> cl/368302 should fix this.

[mkustermann]

Filed cherry-pick request for stable: #55853