I was playing around with multiple type parameters as upper bounds (using

@Suppress("BOUNDS_NOT_ALLOWED_IF_BOUNDED_BY_TYPE_PARAMETER" /* we don't care about java compatibility, hence OK*/)

to suppress the warning. Unfortunately it is not supported that well (which is not a surprise) and intellij sometimes reports rubbish or does not detect type errors. Should I open issues for this?

You should wait until https://youtrack.jetbrains.com/issue/KT-19215/Relax-restriction-on-type-parameters-that-are-constrained-by-other-type-parameters is implemented and then report issues if it doesn’t work correctly.