DDC: Mixing in an implementer of a covariant field or setter results in getter that always returns null

[greglittlefield-wf]

Mixing in an implementer of a covariant field or setter and then concretely implementing that member with another class can result in the target class's getter always returning null in DDC-compiled code. Potentially a similar issue to #29914 (see below).

Reduced test case:

void main() {
  // Prints 'null', not 'foo from BaseClass' as expected, or even 'foo from Interface'.
  print('SubClass().foo');
  print(SubClass().foo);

  // Setting the value works as expected, though; 
  // this prints 'set foo in BaseClass' as expected.
  SubClass().foo = 'bar';
}

class Interface {
  // This can be a getter and covariant setter or a covariant field.
  covariant Object foo = 'foo from Interface';
}

mixin Mixin implements Interface {}

class BaseClass {
  // This can be a getter/setter or another field.
  Object get foo => 'foo from BaseClass';
  set foo(Object value) {
    print('set foo in BaseClass');
  }
}

// BaseClass.foo satisfies the need to concretely implement Interface.foo when mixed in.
// This issue can also be reproduced by mixing in BaseClass instead of extending it.
class SubClass extends BaseClass with Mixin {}

I noticed that the issue does not occur if you tweak this code by either:

• Adding implements Interface to BaseClass
• Moving implements Interface from Mixin to BaseClass
• Making Interface.foo non-covariant

When logging this class to the console, you can see that there's a setter declared in one of the objects in the prototype chain without a corresponding getter. This seems like a similar issue to #29914, so perhaps the fix is also something similar?

Dart SDKs I've checked in which the issue occurs:

• Dart 2.14.0-377.0.dev (I only tried with null safety enabled)
• Dart 2.13.3 (both with and without null safety enabled)
• Dart 2.12.0 (I only tried without null safety enabled)

Dart SDKs I've checked in which the issue does not occur:

• Dart 2.10.4
• Dart 2.9.0
• Dart 2.7.2

[nshahan]

@greglittlefield-wf Thanks for the minimal reproduction and the detailed investigation. I think I see what the bug is here and I'm testing a fix. The setter with a covariant parameter is causing the synthetic mixin class in the hierarchy to generate a forwarding setter. It should have a matching forwarding getter because there is a getter higher up the class hierarchy but it's missing.

[greglittlefield-wf]

@nshahan Np, thank you for fixing this issue so quickly!!