Gradle cannot compile after generated code on Custom Language

I'm following the Custom Language Plugin Tutorial, on part 4 when you generate some code from bnf and flex files and mark generated folders as generated source root. Gradle cannot recognise them and can't compile on runIde task. Everything seems to be fine, no error codes, IDEA shows up all code as OK but when I try to run the plugin by runIde task I  get the errors. I have no ide how to make gradle compiles both code (yours and generated one).