Do Java 8 default methods break source compatibility?

Question

It has generally been the case the Java source code has been forward compatible. Until Java 8, as far as I know, both compiled classes and source have been forward

Answer 1

Doesn't JDK 1.8 introduce a forward incompatibility for Java source code due to default methods?

Yes as you've seen your self.

Is this the first such forward incompatible change?

No. Java 5 enumkeyword was also breaking because before that you could have variables named that which would no longer compile in Java 5 +

Was this considered or discussed when default methods where designed and implemented? Is it documented anywhere?

Yes Orcale Java 8 source incompatibility description

Was the (admittedly small) inconvenience discounted versus the benefits?

Yes