dart2js miscompilation of `late final` fields when optimizations are disabled

[simolus3]

Consider this program:

void main() {
  final x = X();
  print('should be test');
  print(x.name);
}

final class X {
  late final String name = 'test';
}

Now, compile it with dart compile js -O0 --no-minify repro.dart. We'd expect this to print should be test and test, but:

$ node out.js
should be test
Instance of 'JavaScriptObject'

Relevant snippets from the generated code:

  var $ = {};

  A = {JS_CONST: function JS_CONST() {
    },
    printToConsole(line) {
      A.printString(line);
    },
    isSentinel(value) {
      return false; // ?????
    },

    X: function X() {
      this.__X_name_FI = $;
    },

  A.X.prototype = {
    get$name() {
      var value = this.__X_name_FI;
      if (A.boolConversionCheck(A.isSentinel(value))) {
        this.__X_name_FI !== $ && A.throwLateFieldADI("name");
        value = this.__X_name_FI = "test";
      }
      return value;
    }
  };

It looks like the definition for isSentinal should be value === $, but it's not. The broken isSentinal implementation then causes all late final fields to return the sentinel value, breaking Dart programs in weird ways.
This happens both on Dart 3.6.2 and a recent build from main (I've tested with 3.8.0-edge.1c24e58896b6706999f55b269f2477d393d97fab).

[simolus3]

This fixes the problem, but I'm not sure if that's the right place to put it.

diff --git a/pkg/compiler/lib/src/inferrer/types.dart b/pkg/compiler/lib/src/inferrer/types.dart
index 7e393f2d3f1..d8fe7d3a892 100644
--- a/pkg/compiler/lib/src/inferrer/types.dart
+++ b/pkg/compiler/lib/src/inferrer/types.dart
@@ -621,6 +621,14 @@ class TrivialGlobalTypeInferenceResults implements GlobalTypeInferenceResults {
 
   @override
   AbstractValue resultOfParameter(Local parameter) {
+    var result = _trivialParameterResult;
+
+    if (parameter is JLocal) {
+      if (closedWorld.commonElements.isIsSentinel(parameter.memberContext)) {
+        return closedWorld.abstractValueDomain.includeLateSentinel(result);
+      }
+    }
+
     return _trivialParameterResult;
   }

[fishythefish]

Thanks for the report! We've seen this in a couple other issues (#49599 and #55058) but it hasn't been high priority to investigate/fix these since -O0 gets virtually zero use. I'll consolidate these issues and work on sending out a fix.

[simolus3]

Thanks (and sorry about the duplicate issue then). As someone who needs to use dart2js (I'm writing web workers in Dart which can't load the modules emitted by dartdevc that easily), is there a workaround for this? E.g. -O0 with type inference manually enabled?

[fishythefish]

Unfortunately, I'm not sure if there is an easy workaround. -O0 essentially expands into --disable-type-inference + some other stuff, but there's no flag to counteract that. Rather than adding such a flag, I'll try to just implement the fix ASAP.

[fishythefish]

Actually, there may be a workaround depending on how much control you have over the compiler invocation. -O0 is equivalent to --disable-inlining --disable-type-inference --disable-rti-optimization. So rather than passing -O0, you could just pass the first and third flags.