scala3-UniversalApply2.scala

scala> case class Person(name: String, age: Int):
     |   def this(name:String) = this(name, 18)
// defined case class Person

scala> Person("Dean", 100)
val res2: Person = Person(Dean,100)

scala> Person("Dean")
1 |Person("Dean")
  |^^^^^^^^^^^^^^
  |missing argument for parameter age of method apply: (name: String, age: Int): Person

scala> new Person("Dean")
val res3: Person = Person(Dean,18)