Fix #4535: Handle Null and Nothing in completions

Do not complete on Nothing and complete as AnyRef for Null.

Add missing Artifact to remove `$isInstanceOf` completion and
workaround missing decoding for `$u2192` to `→`.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -270,7 +270,7 @@ class Definitions {
     lazy val Any_getClass: TermSymbol = enterMethod(AnyClass, nme.getClass_, MethodType(Nil, ClassClass.typeRef.appliedTo(TypeBounds.empty)), Final)
     lazy val Any_isInstanceOf: TermSymbol = enterT1ParameterlessMethod(AnyClass, nme.isInstanceOf_, _ => BooleanType, Final)
     lazy val Any_asInstanceOf: TermSymbol = enterT1ParameterlessMethod(AnyClass, nme.asInstanceOf_, _.paramRefs(0), Final)
-    lazy val Any_typeTest: TermSymbol = enterT1ParameterlessMethod(AnyClass, nme.isInstanceOfPM, _ => BooleanType, Final | Synthetic)
+    lazy val Any_typeTest: TermSymbol = enterT1ParameterlessMethod(AnyClass, nme.isInstanceOfPM, _ => BooleanType, Final | Synthetic | Artifact)
       // generated by pattern matcher, eliminated by erasure
 
     def AnyMethods: List[TermSymbol] = List(Any_==, Any_!=, Any_equals, Any_hashCode,

compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -150,7 +150,9 @@ object Completion {
       nameToSymbols.map { case (name, symbols) =>
         val typesFirst = symbols.sortWith((s1, s2) => s1.isType && !s2.isType)
         val desc = description(typesFirst)
-        Completion(name.toString, desc, typesFirst)
+        val strName = name.toString
+        val label = if (strName == "$u2192") "→" else strName // TODO fix name.decode and use it
+        Completion(label, desc, typesFirst)
       }
     }
 
@@ -207,11 +209,17 @@ object Completion {
      * considered.
      */
     def addMemberCompletions(qual: Tree)(implicit ctx: Context): Unit = {
-      addAccessibleMembers(qual.tpe)
-      if (!mode.is(Mode.Import)) {
-        // Implicit conversions do not kick in when importing
-        implicitConversionTargets(qual)(ctx.fresh.setExploreTyperState())
-          .foreach(addAccessibleMembers)
+      if (qual.tpe =:= defn.NothingType)
+        ()
+      else if (qual.tpe =:= defn.NullType)
+        addAccessibleMembers(defn.AnyRefType)
+      else {
+        addAccessibleMembers(qual.tpe)
+        if (!mode.is(Mode.Import)) {
+          // Implicit conversions do not kick in when importing
+          implicitConversionTargets(qual)(ctx.fresh.setExploreTyperState())
+            .foreach(addAccessibleMembers)
+        }
       }
     }
 

compiler/test/dotty/tools/repl/TabcompleteTests.scala

@@ -103,4 +103,25 @@ class TabcompleteTests extends ReplTest {
     assertEquals(comp.find(_.startsWith("<")), None)
     assert(!comp.contains("package"))
   }
+
+  @Test def `null` = fromInitialState { implicit s =>
+    val comp = tabComplete("null.")
+    assertEquals(
+      List("!=", "##", "==", "asInstanceOf", "clone", "eq", "equals", "finalize", "getClass", "hashCode",
+          "isInstanceOf", "ne", "notify", "notifyAll", "synchronized", "toString", "wait"),
+      comp.distinct.sorted)
+  }
+
+  @Test def anyRef = fromInitialState { implicit s =>
+    val comp = tabComplete("(null: AnyRef).")
+    assertEquals(
+      List("!=", "##", "+", "->", "==", "asInstanceOf", "clone", "ensuring", "eq", "equals", "finalize", "formatted",
+          "getClass", "hashCode", "isInstanceOf", "ne", "notify", "notifyAll", "synchronized", "toString", "wait", "→"),
+      comp.distinct.sorted)
+  }
+
+  @Test def `???` = fromInitialState { implicit s =>
+    val comp = tabComplete("???.")
+    assertEquals(Nil, comp)
+  }
 }