Consider the following example:
Java:
public class J {
public static String foo() { return null; }
}
Kotlin:
fun check(fn: () -> Any) = fn()
fun test() = check { J.foo() }
When a lambda expression returns a value of platform type ('String!'),
corresponding lambda has platform type in its return type, which is
approximated to corresponding nullable type ('String?') in IR.
However, the lambda itself could occur in position with a functional
expected type ('() -> Any'). This implies an extra implicit cast on a
return value of lambda expression ('J.foo()'), although it conforms to
the return type of lambda.
Mikhail Zarechenskiy