match-types.scala

miles@tarski:~% cs launch scala3-repl   
scala> trait A                                                                                                                              
     | trait B
     | type M[X] = X match
     |   case A => Int
     |   case B => String
     | 
// defined trait A
// defined trait B

scala> val x: String = ??? : M[B]                                                                                                           
1 |val x: String = ??? : M[B]
  |                ^^^^^^^^^^
  |                Found:    M[B]
  |                Required: String

scala> class Inv[T]
// defined class Inv

scala> type M[X] = Inv[X] match
     |   case Inv[A] => Int
     |   case Inv[B] => String
     | 

scala> val x: String = ??? : M[B]
scala.NotImplementedError: an implementation is missing
  at scala.Predef$.$qmark$qmark$qmark(Predef.scala:345)
  ... 28 elided

scala>