build.sbt

scalaVersion := "3.0.0"

libraryDependencies += "org.mockito" % "mockito-core" % "3.10.0"

Main.scala

import org.mockito.ArgumentMatchers
import org.mockito.Mockito

object Main {
  def main(args: Array[String]): Unit = {
    val mock = Mockito.mock(classOf[Option[Int]])
    Mockito.when(mock.get).thenReturn("aaa")
    val x = mock.get
    println(x)
  }
}

scala-2-13-16.txt

Main.scala:7:39: type mismatch;
[error]  found   : String("aaa")
[error]  required: Int
[error]     Mockito.when(mock.get).thenReturn("aaa")
[error]                                       ^

scala-3-0-0.txt

[info] running (fork) Main 
[error] Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
[error] 	at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:99)
[error] 	at Main$.main(Main.scala:8)
[error] 	at Main.main(Main.scala)
[error] Nonzero exit code returned from runner: 1
[error] (Compile / run) Nonzero exit code returned from runner: 1

X-print-all-AST.scala

 def main(args: Array[String]): Unit = 
   {
     val mock: Option[Int] = 
       org.mockito.Mockito.mock[Option[Int]](classOf[Option[Int]])
     org.mockito.Mockito.when[Matchable](mock.get).thenReturn("aaa")
     val x: Int = mock.get
     println(x)
   }