Alternative fix based on implicits.Not[=:=[A, B]]

Author: allanrenucci

library/src/dotty/Show.scala

@@ -12,6 +12,8 @@ trait Show[-T] {
   * default instances in this object
   */
 object Show {
+  type =!=[A, B] = implicits.Not[=:=[A, B]]
+
   private[this] val defaultShow: Show[Any] = new Show[Any] {
     def show(x: Any) = x.toString
   }
@@ -71,11 +73,7 @@ object Show {
     }) + "'"
   }
 
-  implicit val nothingShow: Show[Nothing] = new Show[Nothing] {
-    def show(x: Nothing) = ???
-  }
-
-  implicit def showList[T](implicit st: Show[T]): Show[List[T]] = new Show[List[T]] {
+  implicit def showList[T](implicit st: Show[T], ev: T =!= Nothing): Show[List[T]] = new Show[List[T]] {
     def show(xs: List[T]) =
       if (xs.isEmpty) "List()"
       else "List(" + xs.map(_.show).mkString(", ") + ")"
@@ -85,7 +83,7 @@ object Show {
     def show(xs: Nil.type) = "List()"
   }
 
-  implicit def showOption[T](implicit st: Show[T]): Show[Option[T]] = new Show[Option[T]] {
+  implicit def showOption[T](implicit st: Show[T], ev: T =!= Nothing): Show[Option[T]] = new Show[Option[T]] {
     def show(ot: Option[T]): String = ot match {
       case Some(t) => "Some("+ st.show(t) + ")"
       case none => "None"
@@ -96,7 +94,7 @@ object Show {
     def show(n: None.type) = "None"
   }
 
-  implicit def showMap[K,V](implicit sk: Show[K], sv: Show[V]): Show[Map[K,V]] = new Show[Map[K,V]] {
+  implicit def showMap[K,V](implicit sk: Show[K], sv: Show[V], ev1: K =!= Nothing, ev2: V =!= Nothing): Show[Map[K,V]] = new Show[Map[K,V]] {
     def show(m: Map[K, V]) =
       "Map(" + m.map { case (k, v) => sk.show(k) + " -> " + sv.show(v) } .mkString (", ") + ")"
   }