Github user koeninger commented on the issue:
https://github.com/apache/spark/pull/13998
Basically, if you're adding a new method to an interface, it's because you
need new behavior.
This is the fragile base class problem inherent in, well, inheriting
behavior. You can guess at what the intended default behavior should be, and
put in a default method... but that behavior is going to be wrong for some
subclass of users.
The alternative is to actually change the interface (not add a default
method) which makes it obvious at compile time that things have changed. You
can certainly add an abstract method without an implementation to a scala
trait, since it's just an interface. It means people that made a subclass are
forced to think about the new behavior and do the right thing for themselves.
I understand your bias is towards maintaining binary compatibility, I'm
just trying to point out that binary compatibility doesn't actually mean things
are going to work right.
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