When placed on an external interface or class, requires all its child interfaces, classes, and objects to be external as well.

The compiler mangles identifiers of functions and properties from non-external interfaces and classes, and doesn't mangle from external ones. Requiring external interfaces and classes inheritors being external is necessary to avoid non-obvious bugs when identifier naming in an inheritor doesn't correspond to naming in the base class or interface.


// From kotlin-wrappers
external interface Props {
    var key: Key?

// User code
external interface LayoutProps : Props {
    var layout: Layout?

external interface ComponentProps : LayoutProps // OK

external interface ComponentA : ComponentProps // OK

interface ComponentB : ComponentProps // Compilation error!

This annotation is experimental, meaning that the restrictions mentioned above are subject to change.