Fix completion without prefix

When the user writes `qualifier.<tab>`, we get the tree corresponding to
`qualifier.<error>`. When encountering the error, we were not doing any
completion, but we should consider both terms and types in this context.

Author: Duhemm

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

@@ -45,7 +45,7 @@ object Completion {
   private def completionMode(path: List[Tree], pos: SourcePosition): Mode = {
     path match {
       case (ref: RefTree) :: _ =>
-        if (ref.name == nme.ERROR) Mode.None
+        if (ref.name == nme.ERROR) Mode.Term | Mode.Type
         else if (ref.name.isTermName) Mode.Term
         else if (ref.name.isTypeName) Mode.Type
         else Mode.None
@@ -301,7 +301,7 @@ object Completion {
     val Type: Mode = new Mode(2)
 
     /** Both term and type symbols are allowed */
-    val Import: Mode = Term | Type
+    val Import: Mode = new Mode(4) | Term | Type
   }
 
 }

language-server/test/dotty/tools/languageserver/CompletionTest.scala

@@ -166,4 +166,13 @@ class CompletionTest {
           |}""".withSource
       .completion(m1, Set())
   }
+
+  @Test def completeNoPrefix: Unit = {
+    code"""class Foo { def foo = 0 }
+          |object Bar {
+          |  val foo = new Foo
+          |  foo.${m1}
+          |}""".withSource
+      .completion(m1, results => assertTrue(results.nonEmpty))
+  }
 }