kernel: Do not abstract and share mixin applications

[rakudrama]

dart2js is blocked on this issue.

Consider:

class One<R> {}

class AnInterface<X> implements One<int> {}

class ADifferentInterface<Y> extends AnInterface<Y>{}

class TheMixin<Q> implements AnInterface<Q>  {}

class Two<T> extends One<int> with TheMixin<T> implements AnInterface<T> {}

class Three<T> extends One<int> with TheMixin<T> implements ADifferentInterface<T> {}

main() {
  new Two<double>();
}

What we see is that One<int> with TheMixin<T> is abstracted like so

class OneWithTheMixin<U, V> = One<U> with TheMixin<V>;
class Two<T> extends OneWithTheMixin<int, T> ...

This breaks on dart2js. dart2js has a constraint that a class cannot implement a type with different parameterizations. (The constraint allows the relationship between Two and One to be expressed as a function, the existence of this function is fundamental to the whole type representation.)

However, the introduced type OneWithTheMixin implements One<U> and One<int>. In particular, the raw type OneWithTheMixin implements One<dynamic> and One<int>.

The simplest for dart2js would be to create a fresh class for each with. This is what the old pipeline does - so it would be an advantage to do the same (we can compare outputs), and then reconsider a version of this abstraction later when we are done with the transition to Kernel.

This issue is blocking our next step, compiling angular apps with dart2js-with-kernel, so it is our highest priority issue with kernel.

[peter-ahe-google]

Preferred name would be:

"Object with Mixin1, Mixin2 for Subclass"

[peter-ahe-google]

Aske, could you take a look at this?

[peter-ahe-google]

I already took a stab at implementing the semantics dart2js need:

https://dart-review.googlesource.com/c/sdk/+/17882

For this CL to land, we'd need to keep the old behavior and introduce a flag in pkg/kernel/lib/target/targets.dart that we can use to pick the desired implementation.

However, Stephen reported that:

1. language_2/mixin_generic_test fails. The reason appears to be missing type parameters in the asXXX functions. This hints that the mixin application has missing type parameters.

2. I see the following, which confirms the above:

abstract class _C&S&M = self::S<core::Map<self::C::U, self::C::V>> with self::M<core::List<self::C::U>> {
}
abstract class _C&S&M&N = self::_C&S&M with self::N<core::Set<self::C::V>> {
}
class C<U extends core::Object, V extends core::Object> extends self::_C&S&M&N {
  default constructor •() → void
    : super self::S::•()
    ;
}

It appears that the type parameters are missing and there are references to type parameters no longer in scope (this may explain #3)
I believe that all of the mixin applications _C&... should have their own type parameters exactly matching the same types as C.

3. I can't fully print the test. If I try

   ./pkg/front_end/tool/fasta compile --dump-ir --target=none tests/language_2/mixin_generic_test.dart

I get a crash:

Unhandled exception:
NoSuchMethodError: The method '>=' was called on null.
Receiver: null
Tried calling: >=(0)
#0      Object.noSuchMethod (dart:core-patch/dart:core/object_patch.dart:46)
#1      BinaryPrinter.writeUInt30 (package:kernel/binary/ast_to_binary.dart:71:18)
#2      BinaryPrinter.visitTypeParameterType (package:kernel/binary/ast_to_binary.dart:1385:5)
#3      TypeParameterType.accept (package:kernel/ast.dart:4761:34)
#4      BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:148:10)
#5      List.forEach (dart:core-patch/dart:core/growable_array.dart:270)
#6      BinaryPrinter.writeList (package:kernel/binary/ast_to_binary.dart:137:11)
#7      BinaryPrinter.writeNodeList (package:kernel/binary/ast_to_binary.dart:141:5)
#8      BinaryPrinter.visitInterfaceType (package:kernel/binary/ast_to_binary.dart:1338:7)
#9      InterfaceType.accept (package:kernel/ast.dart:4465:34)
#10     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:148:10)
#11     List.forEach (dart:core-patch/dart:core/growable_array.dart:270)
#12     BinaryPrinter.writeList (package:kernel/binary/ast_to_binary.dart:137:11)
#13     BinaryPrinter.writeNodeList (package:kernel/binary/ast_to_binary.dart:141:5)
#14     BinaryPrinter.visitSupertype (package:kernel/binary/ast_to_binary.dart:1349:7)
#15     Supertype.accept (package:kernel/ast.dart:4869:26)
#16     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:148:10)
#17     BinaryPrinter.writeOptionalNode (package:kernel/binary/ast_to_binary.dart:156:7)
#18     BinaryPrinter.visitClass (package:kernel/binary/ast_to_binary.dart:621:5)
#19     Class.accept (package:kernel/ast.dart:825:30)
#20     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:148:10)
#21     List.forEach (dart:core-patch/dart:core/growable_array.dart:270)
#22     BinaryPrinter.writeList (package:kernel/binary/ast_to_binary.dart:137:11)
#23     BinaryPrinter.writeNodeList (package:kernel/binary/ast_to_binary.dart:141:5)
#24     BinaryPrinter.visitLibrary (package:kernel/binary/ast_to_binary.dart:496:5)
#25     Library.accept (package:kernel/ast.dart:390:30)
#26     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:148:10)
#27     List.forEach (dart:core-patch/dart:core/growable_array.dart:270)
#28     BinaryPrinter.writeLibraries (package:kernel/binary/ast_to_binary.dart:337:23)
#29     BinaryPrinter.writeProgramFile (package:kernel/binary/ast_to_binary.dart:219:5)
#30     writeProgramToFile (package:front_end/src/fasta/kernel/utils.dart:42:13)
<asynchronous suspension>
#31     CompileTask.compile (file:///usr/local/google/home/sra/Dart/sdk/pkg/front_end/tool/_fasta/entry_points.dart:164:11)
<asynchronous suspension>
#32     compile.<anonymous closure> (file:///usr/local/google/home/sra/Dart/sdk/pkg/front_end/tool/_fasta/entry_points.dart:98:25)
<asynchronous suspension>
#33     withGlobalOptions.<anonymous closure> (file:///usr/local/google/home/sra/Dart/sdk/pkg/front_end/tool/_fasta/command_line.dart:376:13)
#34     CompilerContext.runInContext.<anonymous closure> (package:front_end/src/fasta/compiler_context.dart:89:33)
#35     _rootRun (dart:async/zone.dart:1124)
#36     _CustomZone.run (dart:async/zone.dart:1021)
#37     runZoned (dart:async/zone.dart:1499)
#38     CompilerContext.runInContext (package:front_end/src/fasta/compiler_context.dart:89:12)
#39     CompilerContext.runWithOptions (package:front_end/src/fasta/compiler_context.dart:96:41)
#40     withGlobalOptions (file:///usr/local/google/home/sra/Dart/sdk/pkg/front_end/tool/_fasta/command_line.dart:369:26)
#41     compile (file:///usr/local/google/home/sra/Dart/sdk/pkg/front_end/tool/_fasta/entry_points.dart:91:18)
<asynchronous suspension>
#42     compileEntryPoint (file:///usr/local/google/home/sra/Dart/sdk/pkg/front_end/tool/_fasta/entry_points.dart:49:11)
<asynchronous suspension>

[peter-ahe-google]

We may be in luck and be able to use the approach taken in https://dart-review.googlesource.com/c/sdk/+/33884

[peter-ahe-google]

Fixed in 9f56e28.