Enable inline classes

I know I read it somewhere but I can’t find it. How do I enable inline classes again using gradle. Right now I just get this error

The feature “inline classes” is experimental and should be enabled explicitly

but I can’t find anywhere how I enable this feature. I guess it is something like you do with coroutines

kotlin { experimental { coroutines 'enable' } }

but I can’t find the required flag for inline classes.

mmm. Could have sworn it was already part of 1.2.5. Sadly 1.2.6 does not work with serialization yet so, no luck for me.