Fix completion mode filtering + optimize scopeCompletions (scala#23172)

I've changed scopeCompletions to lazy val to memoize because I beliveve
we don't need to recompute it. The actual fix is at the comment. There
is no need to create a new Completer just to filter all of remaining
completions at later stage.

Fixes scala#23150

Author: rochala

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

@@ -242,9 +242,7 @@ object Completion:
       case tpd.Select(qual @ tpd.This(_), _) :: _ if qual.span.isSynthetic      => completer.scopeCompletions.names
       case StringContextApplication(qual) =>
         completer.scopeCompletions.names ++ completer.selectionCompletions(qual)
-      case tpd.Select(qual, _) :: _               if qual.typeOpt.hasSimpleKind =>
-        completer.selectionCompletions(qual)
-      case tpd.Select(qual, _) :: _                                             => Map.empty
+      case tpd.Select(qual, _) :: _                                             => completer.selectionCompletions(qual)
       case (tree: tpd.ImportOrExport) :: _                                      => completer.directMemberCompletions(tree.expr)
       case _                                                                    => completer.scopeCompletions.names
 
@@ -367,7 +365,7 @@ object Completion:
    *  For the results of all `xyzCompletions` methods term names and type names are always treated as different keys in the same map
    *  and they never conflict with each other.
    */
-  class Completer(val mode: Mode, pos: SourcePosition, untpdPath: List[untpd.Tree], matches: Name => Boolean):
+  class Completer(val mode: Mode, pos: SourcePosition, untpdPath: List[untpd.Tree], matches: Name => Boolean)(using Context):
     /** Completions for terms and types that are currently in scope:
      *  the members of the current class, local definitions and the symbols that have been imported,
      *  recursively adding completions from outer scopes.
@@ -381,7 +379,7 @@ object Completion:
      *    (even if the import follows it syntactically)
      *  - a more deeply nested import shadowing a member or a local definition causes an ambiguity
      */
-    def scopeCompletions(using context: Context): CompletionResult =
+    lazy val scopeCompletions: CompletionResult =
 
       /** Temporary data structure representing denotations with the same name introduced in a given scope
        *  as a member of a type, by a local definition or by an import clause
@@ -475,9 +473,15 @@ object Completion:
     def selectionCompletions(qual: tpd.Tree)(using Context): CompletionMap =
       val adjustedQual = widenQualifier(qual)
 
-      implicitConversionMemberCompletions(adjustedQual) ++
+      if qual.symbol.is(Package) then
+        directMemberCompletions(adjustedQual)
+      else if qual.typeOpt.hasSimpleKind then
+        implicitConversionMemberCompletions(adjustedQual) ++
         extensionCompletions(adjustedQual) ++
         directMemberCompletions(adjustedQual)
+      else
+        Map.empty
+
 
     /** Completions for members of `qual`'s type.
      *  These include inherited definitions but not members added by extensions or implicit conversions
@@ -584,8 +588,7 @@ object Completion:
       // There are four possible ways for an extension method to be applicable
 
       // 1. The extension method is visible under a simple name, by being defined or inherited or imported in a scope enclosing the reference.
-      val termCompleter = new Completer(Mode.Term, pos, untpdPath, matches)
-      val extMethodsInScope = termCompleter.scopeCompletions.names.toList.flatMap:
+      val extMethodsInScope = scopeCompletions.names.toList.flatMap:
         case (name, denots) => denots.collect:
           case d: SymDenotation if d.isTerm && d.termRef.symbol.is(Extension) => (d.termRef, name.asTermName)
 

presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala

@@ -2203,3 +2203,28 @@ class CompletionSuite extends BaseCompletionSuite:
         |""".stripMargin,
       ""
     )
+
+  @Test def `no-completions-on-package-selection` =
+    check(
+      """package one.@@
+        |""".stripMargin,
+      ""
+    )
+
+  @Test def `no-extension-completion-on-packages` =
+    check(
+      """object M:
+        |  scala.runt@@
+        |""".stripMargin,
+      """runtime scala
+        |PartialFunction scala""".stripMargin // those are the actual members of scala
+    )
+
+  @Test def `no-extension-completions-on-package-objects` =
+    check(
+      """package object magic { def test: Int = ??? }
+        |object M:
+        |  magic.@@
+        |""".stripMargin,
+      "test: Int"
+    )