From the announcement of 1.3.40 Early Access Preview
Early Access Preview for Kotlin 1.3.40 is now open. We begin stabilization process of new type inference . Now new type inference , which has been experimental since 1.3.0, is enabled by default in the plugin, but not for compilation. Thus, if code without errors in IDE does not compile, please, report the issue to our issue tracker . The feature can be disabled by unchecking Enable new type inference algorithm for IDE analysis (experimental) checkbox in Settings → Build, Execution, Deployment → Compiler → Kotlin Compiler .