Kernel's VM specific mixin transformation generates classes with incompatible overrides

[mraleph]

When I compile the following Dart code below:

abstract class I<T> {
  void map(void f(T e));
}


class A<T> {
  void map(void f(T e)) {

  }
}

abstract class B implements I<int> {
}

class Good extends A<int> with B implements I<int> {
}

In Kernel I get:

  abstract class _A&B^#U0^<#U0 extends core::Object> extends y::A<y::_A&B^#U0^::#U0> implements y::B {
    constructor •() → void
      : super y::A::•()
      ;
  }
  class Good extends y::_A&B^#U0^<core::int> implements y::I<core::int> {
    default constructor •() → void
      : super y::_A&B^#U0^::•()
      ;
  }

The artificial class introduced by mixin transformation actually introduces incompatible override: A<#U0> provides A<#U0>.map of type ((#U0) -> void) -> void, but implements y::B requires map of type ((int) -> void) -> void. These types are incompatible.

[kmillikin]

That's not generated by the transformation, it was in the program before transformation:

abstract class _A&B^#U0^<#U0 extends core::Object> = self::A<self::_A&B^#U0^::#U0> with self::B {
}
class Good extends self::_A&B^#U0^<core::int> implements self::I<core::int> {
  default constructor •() → void
    : super self::A::•()
    ;
}

I think instead we'd like to have something like this for the unnamed mixin application:

abstract class _A&B = self::A<core::int> with self::B {
}
class Good extends self::_A&B^#U0^<core::int> implements self::I<core::int> {
  default constructor •() → void
    : super self::A::•()
    ;
}

@peter-ahe-google

[peter-ahe-google]

@mraleph as we discussed offline, if you use a named mixin application this doesn't happen.

The root cause of this is how Fasta canonicalizes unnamed mixin applications, and we generally seem to want to keep the canonicalization as this has a positive effect on the number of classes generated overall.

Obviously, we need to fix this somehow, but it is not a high priority at the moment.