The Map interface in Kotlin (using V1.6.21) has a signature of

interface Map

Why is K invariant instead of covariant (out K)?

The documentation of type parameter K says:

The map is invariant in its key type, as it can accept key as a parameter (of containsKey for example) and return it in keys set.

However, interface Set is covariant in the element type, so the the last part ("return it in keys set") is not applicable, at least not immediately.

Further, the type parameter K is used only at occurrences where the map state is not modified, for lookup purposes (methods containsKey, get, getOrDefault). At these places, isn't it safe to use @UnsafeVariance? After all, that same technique was employed to Map's value type parameter V, for example in containsValue, to allow making V covariant.