BoxedUnit cannot be cast to class scala.Function0

Compiler version

Minimized code

Welcome to Scala 3.1.3-RC1-bin-SNAPSHOT-git-d38e10e (17.0.2, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> def g(args: Int*)(block: => Unit): Unit = println(s"Well, ${args.sum} and $block")
def g(args: Int*)(block: => Unit): Unit

scala> def f = g()
def f: (=> Unit) => Unit

scala> f(println("x"))
x
java.lang.ClassCastException: class scala.runtime.BoxedUnit cannot be cast to class scala.Function0 (scala.runtime.BoxedUnit and scala.Function0 are in unnamed module of loader java.net.URLClassLoader @535b8c24)
  at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
  at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
  ... 33 elided

Expectation

Welcome to Scala 2.13.8 (OpenJDK 64-Bit Server VM, Java 17.0.2).
Type in expressions for evaluation. Or try :help.

scala> def g(args: Int*)(block: => Unit): Unit = println(s"Well, ${args.sum} and $block")
def g(args: Int*)(block: => Unit): Unit

scala> def f = g()
                ^
       error: missing argument list for method g
       Unapplied methods are only converted to functions when a function type is expected.
       You can make this conversion explicit by writing `g _` or `g(_)(_)` instead of `g`.

scala> def f = g() _
def f: (=> Unit) => Unit

scala> f(println("x"))
x
Well, 0 and ()

[som-snytt]

I confirmed it also compiles under separate compilation, so I’ll try to see what’s wrong with REPL.

[odersky]

It only happens in the repl of main as far as I can see. The following program works as expected:

object l1:
  def g(args: Int*)(block: => Unit): Unit = println(s"Well, ${args.sum} and $block")
object l2:
  import l1.*
  def f = g()
@main def Test =
  import l1.*
  import l2.*
  f(println("x"))