Allow multiple bases with default
implementations as long as there's a
non-abstract symbol from a class.
Our rules for Kotlin are stricter than
those in Java.
Check all members for `Visibility.Unknown`,
otherwise we miss them when they come
from supertypes. This is the reason why
the FP intellij build failed with a
cryptic stacktrace instead of a
human-readable diagnostic.
Also, do report the diagnostic at all
cases of `Visibilities.Unknown`. Turns
out, there are no "simple to reason
about" situations here :(
Also, an interesting detail:
`retrieveDirectOverriddenOf` returns an
empty list for intersection overrides.
But this doesn't seem to break anything...
Replacing `CANNOT_INFER_VISIBILITY`'s
type `KtDeclaration` with
`PsiNameIdentifierOwner` and the related
changes in `PositioningStrategies`
were needed to prevent an exception saying that
`PsiClassImpl` is not a subtype of
`KtDeclaration`.
This code does lead to inferring
`Visibilities.Unknown`, but it's not
reported anywhere. This is because it
happens inside `AbstractIntSet` which
comes to us "pre-compiled" by javac,
whereas `FirOverrideChecker` only checks
members within `KotlinClass`.
^KT-65972
Nikolay Lunyak