Perhaps this is an esoteric question, since I will never distribute the code in question and I can fairly easily avoid using GPL code, but I'm curious and it's bugging me--but not enough to consult a lawyer. What is more, I think I am more interested in the morality of this issue than the legality.
I have a program that I use under the CDDL. The program had a pretty small user base to start with and the creators now offer a later version, which is closed source. Thus I have no interest in contributing my minor modification back to the project. For my use only, I would like to include a single module that is GPL to avoid a small amount of tedious coding.
If I were to distribute the result, it is clearly a violation of the terms per various sources such as the FSF's comments on GNU GPL and CDDL.
If I have correctly tracked the source of the incompatibility back to the MPL from the link from wikipedia, all of the issues seem to arise with clauses that occur with distribution. This would imply to me that I can legally modify and combine GPL and CDDL, but cannot legally distribute the combination.
Setting aside the legality, does anyone see any moral issues with making a minor tweak to a program for my own use that introduces this incompatibility for distribution?