Declaration-site variance

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.

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