Fix #4301: Show null as "null"

compiler/test-resources/repl/i4301

@@ -0,0 +1,4 @@
+scala> List(null)
+val res0: List[Null] = List(null)
+scala> List[String](null)
+val res1: List[String] = List(null)

library/src/dotty/Show.scala

@@ -16,8 +16,7 @@ object Show extends LowPrioShow {
     * any `T`, we default to `T#toString`.
     */
   implicit class ShowValue[V](val v: V) extends AnyVal {
-    def show(implicit ev: Show[V]): String =
-      ev.show(v)
+    def show(implicit ev: Show[V]): String = if (v == null) "null" else ev.show(v)
   }
 
   /** Adds escaping backslashes in a string so that it could be put in source code.

library/test/dotty/ShowTests.scala

@@ -53,6 +53,8 @@ class ShowTests {
 
   @Test def showOptions = {
     assertEquals("None", None.show)
+    val empty = Option.empty
+    assertEquals("None", empty.show)
     assertEquals("None", (None: Option[String]).show)
     assertEquals("Some(\"hello opt\")", Some("hello opt").show)
   }
@@ -65,8 +67,6 @@ class ShowTests {
   @Test def withoutShow = {
     case class Car(model: String, manufacturer: String, year: Int)
     assertEquals("Car(Mustang,Ford,1967)", Car("Mustang", "Ford", 1967).show)
-    assertEquals("Map()", Map[Nothing, Nothing]().show)
-    assertEquals("List()", List().show)
   }
 
   @Test def partialShow = {
@@ -79,4 +79,16 @@ class ShowTests {
     assertEquals("Array(1)", Array(1).show)
     assertEquals("Array(1, 2, 3)", Array(1, 2, 3).show)
   }
+
+  @Test def showNull = {
+    assertEquals("null", (null: String).show)
+    assertEquals("List(null)", List(null).show)
+    assertEquals("List(null)", List[String](null).show)
+  }
+
+  @Test def showNothing = {
+    val emptyMap = Map()
+    assertEquals("Map()", emptyMap.show)
+    assertEquals("List()", List().show)
+  }
 }