[cfe/vm] Soundness issue in the vm

[eernstg]

Consider the following program:

class A {
  num foo(num n) {
    print(n.runtimeType);
    return 1.1;
  }
}

abstract class B<X> {
  X foo(X x);
}

class C extends A with B<num> {}

void main() {
  B<Object> b = C();
  b.foo(true);
}

Using tools from dea0488:

The program is accepted by the analyzer (which is OK), and it is accepted by the CFE (which is OK, but the generated kernel code does not include an implicitly induced type checking stub for foo in C, and that's probably a bug).

However, when the program is executed on the vm there is no dynamic error, it just prints 'bool', and that shows that we've encountered a soundness violation.

I'll mark this issue as 'area-front-end', because the generated kernel code seems to be wrong (I would have expected _C&A&B or C to contain an implementation of foo where there required dynamic type check on the actual argument is performed):

abstract class _C&A&B extends self::A implements self::B<core::num> /*isAnonymo>
  synthetic constructor •() → self::_C&A&B
    : super self::A::•()
    ;
  abstract method foo(generic-covariant-impl core::num x) → core::num;
}
class C extends self::_C&A&B {
  synthetic constructor •() → self::C
    : super self::_C&A&B::•()
    ;
}

However, dart2js does raise the dynamic error, so maybe the backends are responsible for part of the work?

[eernstg]

About the degree to which this is covered by the language specification: Cf. #46389 (comment) and dart-lang/language#1729.