Unchanged parent class reported as having a missing method

[2m]

v1

abstract class A { def foo: Int }
class C extends A { def foo: Int = 1 }

v2

abstract class A { def foo: Int }
class B extends A { def foo: Int = 1 }
class C extends B

problem

Fails with:

[error]         The following problems were not expected
[error]         - abstract method foo()Int in class A does not have a correspondent in new version

but class A is unchanged...

expected

No error.

Originally found in akka/akka#24625 (review)

A failing test-case was provided in #204

[BalmungSan]

For the record, this happened as well in cats-effect: typelevel/cats-effect#4424
We don't want to add the recommended filters because that would allow us to actually break binary compatibility by accident.