Fasta accepts conflicting interface type arguments when contravariant

[askeksa-google]

class I<T> {}

class A {}

class B extends A {}

class X implements I<B> {}

class Y extends X implements I<A> {}

Is invalid, since Y implements two different generic instantiations of I. But Fasta does not report an error. If A and B are swapped, an error is correctly reported.

[kmillikin]

This'll be fixed by https://dart-review.googlesource.com/c/sdk/+/76141

As far as I can tell, the code that allows this case was to deal with VM-style super mixins, and probably before mixin type inference was implemented.

[kmillikin]

Right, it was this P0 issue affecting Flutter: #31656

The special case was a "short-term hack" https://dart-review.googlesource.com/c/sdk/+/33884

[kmillikin]

Fixed by c9b05d3