Scala 0.asInstanceOf[B]

code.scala

// Code example for https://stackoverflow.com/q/66989225/5091346

package com.github.aaabramov

import java.time.LocalDateTime

object Test extends App {

  trait TestTrait[A] {

    val data: Seq[A]

    def reduceLeft[B >: A](op: (B, A) => B): B = {

      object reducer extends Function1[A, Unit] {
        var first = true
        var acc: B = 0.asInstanceOf[B]

        override def apply(x: A): Unit =
          if (first) {
            acc = x
            first = false
          }
          else acc = op(acc, x)
      }
      data foreach reducer
      reducer.acc
    }
  }

  val res = new TestTrait[String] {
    override val data: Seq[String] = Seq("1", "2", "3")
  }.reduceLeft(_ + _)

  println(res)                    // 123

  def foo[A]: A = 1.asInstanceOf[A]

  println(foo[String])                 // 1
  println(foo[LocalDateTime])          // 1
  println(foo[LocalDateTime].getClass) // java.lang.Integer


  println(0.asInstanceOf[String]) // Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String

}