Example
Declaration-site variance can be thought of as declaration of use-site variance once and for all the use-sites.
class Consumer<in T> { fun consume(t: T) { ... } }
fun charSequencesConsumer() : Consumer<CharSequence>() = ...
val stringConsumer : Consumer<String> = charSequenceConsumer() // OK since in-projection
val anyConsumer : Consumer<Any> = charSequenceConsumer() // Error, Any cannot be passed
val outConsumer : Consumer<out CharSequence> = ... // Error, T is `in`-parameter
Widespread examples of declaration-site variance are List<out T>, which is immutable so that T only appears as the return value type, and Comparator<in T>, which only receives T as argument.
