WASM error message inconsistency for sublist when range is invalid

[d-markey]

Error messages are inconsistent with Dart2Wasm when calling List.sublist() with an invalid range.

I have implemented a custom list and wanted to check my implementation is up to par with lists from Dart's runtime, but came across this weird behavior when checking for error consistency.

Here's a test repro sublist_wasm_bug_test.dart:

import 'package:test/test.dart';

void main() {
  test('sublist bug', () {
    final list = [1, 2, 3, 4, 5, 6, 7];
    final clist = list.cast<int>();

    try {
      list.sublist(4, 3);
    } catch (ex) {
      print('list --> $ex');
    }

    try {
      clist.sublist(4, 3);
    } catch (ex) {
      print('clist --> $ex');
    }
  });
}

Running with dart test .\test\sublist_wasm_bug_test.dart, output is the following:

00:01 +0: [VM, Kernel] sublist bug
list --> RangeError (end): Invalid value: Not in inclusive range 4..7: 3
clist --> RangeError (end): Invalid value: Not in inclusive range 4..7: 3
00:08 +1: [Chrome, Dart2Js] loading test/sublist_wasm_bug_test.dart
Compiled 11,427,586 input bytes (6,447,067 characters source) to 1,063,150 characters JavaScript in 5.67 seconds using N/A MB of memory

00:10 +1: [Chrome, Dart2Js] sublist bug
list --> RangeError (end): Invalid value: Not in inclusive range 4..7: 3
clist --> RangeError (end): Invalid value: Not in inclusive range 4..7: 3
00:16 +2: [Chrome, Dart2Wasm] loading test/sublist_wasm_bug_test.dart
Generated wasm module 'C:\Users\DMA\AppData\Local\Temp\dart_test_368eb21c\test_f43ba83d\sublist_wasm_bug_test.dart.browser_test.dart.wasm', and JS init file 'C:\Users\DMA\AppData\Local\Temp\dart_test_368eb21c\test_f43ba83d\sublist_wasm_bug_test.dart.browser_test.dart.mjs'.

00:18 +2: [Chrome, Dart2Wasm] sublist bug
list --> RangeError (start): Invalid value: Not in inclusive range 0..7: 4
clist --> RangeError (end): Invalid value: Not in inclusive range 4..7: 3
00:18 +3: All tests passed!

WASM output is inconsistent:

list --> RangeError (start): Invalid value: Not in inclusive range 0..7: 4
clist --> RangeError (end): Invalid value: Not in inclusive range 4..7: 3

Test config dart_test.yaml:

platforms:
  - vm
  - chrome

compilers:
  - dart2js
  - dart2wasm

chain_stack_traces: true
concurrency: 1

I'm running on a Dart runtime provided by the latest version of Flutter.

C:\> dart --version
Dart SDK version: 3.8.1 (stable) (Wed May 28 00:47:25 2025 -0700) on "windows_x64"

C:\ flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, 3.32.5, on Microsoft Windows [version 10.0.26100.4349], locale fr-FR)
[√] Windows Version (11 Professionnel 64-bit, 24H2, 2009)
[√] Android toolchain - develop for Android devices (Android SDK version 33.0.1)
[√] Chrome - develop for the web
[√] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.14.5)
[!] Android Studio (version 2021.1)
    X Unable to determine bundled Java version.
[!] Android Studio (version 2023.3)
    X Unable to find bundled Java version.
[√] Android Studio (version 2024.2)
[√] VS Code (version 1.101.2)
[√] Connected device (3 available)
[√] Network resources

! Doctor found issues in 2 categories.

[mraleph]

cc @mkustermann

[mkustermann]

This is due the code here:

    if (end.ltU(start)) {
      _throwRangeError(start, 0, length, name);
    }

[osa1]

This is because that code checks two error conditions in one instruction, but for proper errors we would need to check those two conditions separately.

It checks these two conditions:

• start < 0
• end < start

Ideally, error reporting for the first error would be

_throwRangeError(start, 0, length, name)

and for the second would be:

_throwRangeError(end, start, length, name)

If we want to keep the fast path while giving better errors we can do:

diff --git a/sdk/lib/_internal/wasm/lib/error_utils.dart b/sdk/lib/_internal/wasm/lib/error_utils.dart
index 20828ea0db0..a81639800f9 100644
--- a/sdk/lib/_internal/wasm/lib/error_utils.dart
+++ b/sdk/lib/_internal/wasm/lib/error_utils.dart
@@ -92,7 +92,11 @@ class RangeErrorUtils {
     assert(length >= 0);
     end ??= length;
     if (end.ltU(start)) {
-      _throwRangeError(start, 0, length, name);
+      if (start < 0) {
+        _throwRangeError(start, 0, length, name);
+      } else {
+        _throwRangeError(end, start, length, name);
+      }
     }
     if (length.ltU(end)) {

But this function is always inlined, so the code size will suffer.

Maybe we could throw as before with minification, and throw different errors with the diff shown above when not minifying. Thoughts @mkustermann?