object Test {

  abstract class Number
  case class MyInt(n: Int) extends Number
  case class MyDouble(d: Double) extends Number

  trait Term[a]
  case class Cell[a](var x: a) extends Term[a]
  final case class NumTerm(val n: Number) extends Term[Number]

  def f[A](t: Term[A], c: Cell[A]): Unit = {
    t match {
      case NumTerm(n) => c.x = MyDouble(1.0)
    }
  }

  val x: Term[Number] = NumTerm(MyInt(5))

  def main(args: Array[String]): Unit = {
    val cell = Cell[Number](MyInt(6))
    Console.println(cell)
    f[Number](new NumTerm(MyInt(5)), cell)
    Console.println(cell)
  }
}

object Test {

  abstract class Number
  case class MyInt(n: Int) extends Number
  case class MyDouble(d: Double) extends Number

  trait Term[a]
  case class Cell[a](var x: a) extends Term[a]
  final case class NumTerm(val n: Number) extends Term[Number]

  def f[A](t: Term[A], c: Cell[A]): Unit = {
    t match {
      case NumTerm(n) => c.x = MyDouble(1.0)  // problem is: this assignment is not type correct, since gadt variable is forgotten
    }
  }

  val x: Term[Number] = NumTerm(MyInt(5))

  def main(args: Array[String]): Unit = {
    val cell = Cell[Number](MyInt(6))
    Console.println(cell)
    f[Number](new NumTerm(MyInt(5)), cell)
    Console.println(cell)
  }
}

--- 
+++ 
@@ -10,7 +10,7 @@
 
   def f[A](t: Term[A], c: Cell[A]): Unit = {
     t match {
-      case NumTerm(n) => c.x = MyDouble(1.0)  // problem is: this assignment is not type correct, since gadt variable is forgotten
+      case NumTerm(n) => c.x = MyDouble(1.0)
     }
   }
 
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