Inline classes are actually a feature of an upcoming 1.3 (Inline classes · Issue #104 · Kotlin/KEEP · GitHub), which is being developed in the master branch under a separate compiler argument for simplicity. You can enable the feature with the compiler argument -XXLanguage:+InlineClasses. Note that this will mark your compiled code as “pre-release”, so that it’d be impossible to use it as a library when using “release” Kotlin versions (less than 1.3 at the moment).