VM: Very confusing error on invalid const assignment

[matanlurey]

The following code:

import 'package:test/test.dart';

class Provider {
  const Provider();
}

class Module {
  final List<Provider> providers;
  const Module(this.providers);
}

const aGoodModule = const Module(const [
  const Provider(),
]);

const listOfThings = const [
  const [],
  const Provider(),
];

const aBadModule = const Module(listOfThings);

void main() {
  test('$aBadModule should be invalid', () {
    expect(aBadModule.providers, const isInstanceOf<List<Provider>>());
  });
}

... throws the following very scary error message:

Unhandled exception:
'file:///Users/matanl/github/dart.scratch/dart2_const_downcast/test/dart2_const_downcast_test.dart': error: Not a constant expression: _ImmutableList len:2 is not an instance of Type: class 'List', args:[TypeArguments: (@0x11e2dc031 H39565ba6) [Type: class 'Provider']]'file:///Users/matanl/github/dart.scratch/dart2_const_downcast/test/dart2_const_downcast_test.dart': error: line 26 pos 10: Not a constant expression.
  test('$aBadModule should be invalid', () {

... this is an error (listOfThings is not a List<Provider>), but it's very hard to figure out here:

• _ImmutableList isn't something I wrote
• This seems to not have the better error messages I've seen elsewhere
• This seems to leak internals of the VM (TypeArguments: (@0x11e2dc031 H39565ba6)).

I'll file a separate bug on the analyzer/DDC - I wouldn't have gotten that far if the IDE warned me.

[matanlurey]

Interestingly, this does fails nicely at compile-time in Dart2JS with --preview-dart-2:

test/dart2_const_downcast_test.dart:21:26:
Error: `listOfThings` of type 'List<Object>' is not a subtype of List<Provider>.
const aBadModule = const Module(listOfThings);
                         ^
Error: Compilation failed.

[matanlurey]

Code: https://github.com/matanlurey/dart.scratch/tree/master/dart2_const_downcast

You can reproduce:

$ dart --preview-dart-2 test/dart2_const_downcast_test.dart

[a-siva]

A small change in the VM makes the error better

{
   Error& error = Error::Handle();
   if (!result_.IsInstanceOf(type, instantiator_type_arguments,
                             function_type_arguments, &error)) {
+    const AbstractType& rtype = AbstractType::Handle(result_.GetType(Heap::kNew));
+    const String& result_str = String::Handle(rtype.UserVisibleName());
+    const String& type_str = String::Handle(type.UserVisibleName());
     H.ReportError(script_, TokenPosition::kNoSource,
                   "Not a constant expression: %s is not an instance of %s",
-                  result_.ToCString(), type.ToCString());
+                  result_str.ToCString(), type_str.ToCString());
   }
 }

Unhandled exception:
'file:///tmp/test/test/dart2_const_downcast_test.dart': error: Not a constant expression: List<Object> is not an instance of List<Provider>'file:///tmp/test/test/dart2_const_downcast_test.dart': error: line 24 pos 10: Not a constant expression.
  test('$aBadModule should be invalid', () {

Let me check all the places in the VM were we report user visible error messages and change the code to use UserVisibleName() instead of a call to ToCString() which prints an internal name,

[matanlurey]

Thanks!

[a-siva]

Landed https://dart-review.googlesource.com/c/sdk/+/57880, will keep this open to scrub for more cases.

[matanlurey]

@a-siva Worth keeping open still? This looks much better now:

'file:///tmp/vm.dart': error: line 21 pos 33: Not a constant expression: Type 'List<Object>' is not a subtype of type 'List<Provider>'
const aBadModule = const Module(listOfThings);
                                ^
'file:///tmp/vm.dart': error: line 27 pos 3: Not a constant expression.
  aBadModule.providers;
  ^

#0      _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:279:19)
#1      _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:165:12)

[a-siva]

Closing it.