gistfile1.scala

abstract class Foo(x: Int) 

class Bar(x: Int) extends Foo(x)

class Baz(x: Int) extends Foo(x)

object Foo {
  import scala.reflect.runtime.{ universe => ru }
  def create[Nothing >: A <: Foo](x: Int)(implicit tt: ru.TypeTag[A]): A = {
    val mirror = ru.runtimeMirror(getClass.getClassLoader)
    val aClass = ru.typeOf[A].typeSymbol.asClassval 
    val cm = mirror.reflectClass(aClass)
    val aCtor = ru.typeOf[A].declaration(ru.nme.CONSTRUCTOR).asMethod
    val ctor = cm.reflectConstructor(aCtor)
    ctor(x).asInstanceOf[A]
  }
}

class Baraz(x: Int) extends Bar(x)

Foo.create[Baraz](1)