Allow import <ident> to show completions

Previously, there would be no completions after simple `import annot`. Now, we make sure that the completions mode is set correctly for that case.

Additionally, whenever both `java.lang` and `scala` imports conflict we choose scala one.

Author: tgodzik

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

@@ -61,6 +61,8 @@ object Completion {
    */
   def completionMode(path: List[Tree], pos: SourcePosition): Mode =
     path match {
+      case Ident(_) :: Import(_, _) :: _ =>
+        Mode.Import
       case (ref: RefTree) :: _ =>
         if (ref.name.isTermName) Mode.Term
         else if (ref.name.isTypeName) Mode.Type
@@ -211,13 +213,35 @@ object Completion {
         // import a.C
         def isSameSymbolImportedDouble =  denotss.forall(_.denots == first.denots)
 
+        def isScalaPackage(scopedDenots: ScopedDenotations) =
+          scopedDenots.denots.exists(_.info.typeSymbol.owner == defn.ScalaPackageClass)
+
+        def isJavaLangPackage(scopedDenots: ScopedDenotations) =
+          scopedDenots.denots.exists(_.info.typeSymbol.owner == defn.JavaLangPackageClass)
+
+        // For example
+        // import java.lang.annotation
+        //    is shadowed by
+        // import scala.annotation
+        def isJavaLangAndScala =  denotss match
+          case List(first, second) =>
+            isScalaPackage(first) && isJavaLangPackage(second) ||
+            isScalaPackage(second) && isJavaLangPackage(first)
+          case _ => false
+
         denotss.find(!_.ctx.isImportContext) match {
           // most deeply nested member or local definition if not shadowed by an import
           case Some(local) if local.ctx.scope == first.ctx.scope =>
             resultMappings += name -> local.denots
 
           case None if isSingleImport || isImportedInDifferentScope || isSameSymbolImportedDouble =>
             resultMappings += name -> first.denots
+          case None if isJavaLangAndScala =>
+            denotss.foreach{
+              denots =>
+                if isScalaPackage(denots) then
+                  resultMappings += name -> denots.denots
+            }
 
           case _ =>
         }

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

@@ -976,4 +976,13 @@ class CompletionTest {
               ("main", Module, "main")
             )
           )
+
+  @Test def i13623_annotation : Unit =
+    code"""import annot${m1}"""
+          .withSource
+          .completion(m1,
+            Set(
+              ("annotation", Module, "scala.annotation")
+            )
+          )
 }