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kotlinlang

Here is a snippet from definition of KProperty (you can crt-click to see it in IDE): `public interface KProperty&lt;out R&gt;`. It is defined with an `out` type parameter `R`, because it only only returns things of type `R` (it never receives them), so a call like `User::firstName.test(42)` infers the following call type `User::firstName.test&lt;Any&gt;(42)`, becuase `Any` is the nearest common supertype between `Int` and `String` and, because of an `out` parameter, `KMutableProperty&lt;String&gt;` is a subtype of `KMutableProperty&lt;Any&gt;`, so a method `test&lt;Any&gt;`, that is applicable to `KMutableProperty&lt;Any&gt;` is also Ok to use on a `KMutableProperty&lt;String&gt;`, which is a subtype of it.