`LinkedList.add` crashs on web with a cast error

[escamoteur]

hi,
a user of my get_it_mixing reported a strange cast error when he runs the app on the web.
I could reproduce this in the most basic setup where adding an item to a linked list throws an exception on the web but not on other platforms.

You could find the repro project here

https://github.com/escamoteur/linked_list_web_error

[√] Flutter (Channel stable, 2.0.5, on Microsoft Windows [Version 10.0.18363.1500], locale de-DE)

@mraleph

[escamoteur]

It happens also in pure Dart on dart pad

import 'dart:collection';
import 'package:flutter/foundation.dart';

class _WatchEntry<TObservedObject, TValue>
    extends LinkedListEntry<_WatchEntry<Object, Object?>> {}
final _watchList = LinkedList<_WatchEntry<Object, Object?>>();

void main() {
  _WatchEntry<Object, Object?> entry = _WatchEntry<ValueListenable<bool>, bool?>();
  try{
    _watchList.add(entry);
  }catch(e){
    print(e);
  }
}

[renefloor]

@escamoteur I first simplified this example to:

import 'dart:collection';

class SomeType extends LinkedListEntry<SomeType>{}
final _watchList = LinkedList<LinkedListEntry<SomeType>>();

void main() {
  var entry = SomeType();
  try{
    _watchList.add(entry);
  }catch(e){
    print(e);
  }
}

Then thought you don't add an object of type LinkedListEntry, but of SomeType. So this example fixed it for me.

import 'dart:collection';

class SomeType extends LinkedListEntry<SomeType>{}
final _watchList = LinkedList<SomeType>();

void main() {
  var entry = SomeType();
  try{
    _watchList.add(entry);
    print('no problem');
  }catch(e){
    print(e);
  }
}

edit: nvm, the LinkedList<LinkedListEntry<SomeType>> was not in the original example.

[escamoteur]

But _watchEntry extends LinkedListEntry too

[leafpetersen]

Slightly simplified example:

import 'dart:collection';
import 'package:flutter/foundation.dart' as _;

class _WatchEntry<S, T>
    extends LinkedListEntry<_WatchEntry<Object, Object?>> {}
final _watchList = LinkedList<_WatchEntry<Object, Object?>>();

void main() {
  _WatchEntry<Object, Object?> entry = _WatchEntry<bool, bool?>();
  try{
    _watchList.add(entry);
  } catch(e){
    print(e);
  }
  print("Done");
}

This crashes with a type error on dartpad, but if I remove the unused import it succeeds.

The type error that gets printed out is:

Expected a value of type 'LinkedList<_WatchEntry<Object, Object>>?', but got one of type 'LinkedList<_WatchEntry<Object, Object?>>'

The fact that _WatchEntry<Object, Object> is a type that doesn't appear anywhere in this program suggests to me that this is a compiler bug.

cc @sigmundch @rakudrama

[sigmundch]

cc @nshahan

Thanks everyone for helping us narrow this down to a smaller repro!

The removal of the unnecessary import was a good clue! Dartpad for flutter uses DDC, whereas dartpad for non-flutter uses dart2js. I didn't know this, but the dartpad UI seems pretty dynamic and switches the backend depending on whether flutter is imported.

Locally, this program shows the error in ddc:

import 'dart:collection';

class A<S, T> extends LinkedListEntry<A<Object, Object?>> {}

void main() {
  LinkedList<A<Object, Object?>>().add(A<bool, bool?>());
}

And removing dart:collection:

class Entry<E extends Entry<E>>{
  L<E>? _list;
}
class L<E extends Entry<E>> {
  add(E entry) { entry._list = this; }
}

class A<T> extends Entry<A<Object?>> {}

void main() {
  L<A<Object?>>().add(A<bool?>());
}

is enough to trigger the failure.

[sigmundch]

a small tweak to remove some of the indirection:

class Entry<E extends Entry<E>>{
  foo(L<E>? input) {}
}
class L<E extends Entry<E>> {}

class A<S> extends Entry<A<Object?>> {}

void main() {
  A<bool>().foo(L<A<Object?>>());
}

This fails with the error: Expected a value of type 'L<A<Object>>?', but got one of type 'L<A<Object?>>'

An odd piece here is the type L<A<Object>>. You cannot write that type in the program (you get a compile-time error highlighting that A<Object> doesn't comply with the bound of L). So this is likely coming from a place where DDC may be dropping the ? accidentally.

[sigmundch]

The output of DDC seems to be initializing the base type of A<S> incorrectly:

dart.setBaseClass(repro8.A$(S), repro8.Entry$(repro8.A$(core.Object)));

as opposed to:

dart.setBaseClass(repro8.A$(S), repro8.Entry$(repro8.A$(dart.nullable(core.Object))));

[sigmundch]

quick update - it seems that whenever we emit this setBaseClass we are omitting the nullability here: https://github.com/dart-lang/sdk/blob/master/pkg/dev_compiler/lib/src/kernel/compiler.dart#L870

[sigmundch]

Here is a smaller repro illustrating the semantic difference:

class I<T> {}
class F<E extends F<E>> extends I<E> {}
class A<T> extends F<A<Object?>> {} // extending F forces DDC to deferr the superclass and trigger the bug

void main() {
  print(A<bool>() is I<A<Object?>>);
  print(A<bool>() is I<A<Object>>);
}

This prints true/false in the VM and dart2js, but true/true in DDC