IntelliJ doesn't pick up generated .class files

Question

I have implemented an AnnotationProcessor that picks up class annotations that take a string argument. The string argument is an expression in a domain-specific language, which the annotation processor will use to compile a class file.

I create a small test project to try this out. The behaviour I see is this:

1. I can successfully build the project using maven
2. I can successfully run the project from within intellij
3. Despite the project RUNNING in intellij, the generated class is not recognised in the editor ("Cannot resolve class '...'"), and intelli-sense does not work, either.

I've tried to find the issue and found:

• the class file that is being generated is being created in target/classes/package/name/KlassName.class (this is the location that the Filer::createClassFile method picks, I'd have expected this to go to some separate directory though).

• if I'd create a java source file during annotation processing (using Filer::createSourceFile), intellij would have no problem. However, I can't do that, since the compiler is a library that really must create classes directly.

I have two guesses about what a solution might look like:

1. This problem might stem from intellij not looking inside target/classes when type checking in the editor window.
2. The class files should be generated in a separate directory instead. If so, what is the setting to fix that?

I have reproduced this issue using intellij IDEA 2016.2.1 and intellij IDEA 2017.2 EAP.