AbstractMethodError when some return type of base class is changed

[adrian-wang]

If original return type was Any, and we change it to some specific type T, if some derived class stay unchanged with Any:

1. compile together: Everything is OK
2. compile only new base class and other derived class: compile OK, when runtime invoke the function in unchanged and not re-compiled class, there will be this AbstractMethodError.

[adrian-wang]

Current workaround: manually add that class to package code base and re-compile.

[adrian-wang]

This only affects those projects that changes interface(s) of Apache Spark.