If I want to distribute my app under the GNU GPL, I am required by the GPL to distribute (or make available) ALL source code of any binaries I distribute with my app. If I use the IDEA GUI designer (with java source code generation) my code uses the classes in com.intellij.uiDesigner.core, but I don't have the source to those packages, so I can't distribute them with my GPL app!! IntelliJ should either release the code to com.intellij.uiDesigner.core under a GPL compatible license or put a warning somewhere that says "Don't use GUI designer in apps the you want to distribute under the GPL!"
Imagine someone doing a lot of work on a GPL project using the GUI designer and then realizing they can't distribute their work.
If someone knows a way (loophole) to legally use the GUI designer in a GPL app let me know!