Fix extension method search in implicit scope

The case where a scope contained only extension methods but no
other givens was mishandled.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -236,8 +236,9 @@ object Implicits {
       }
 
 
-      if (refs.isEmpty) Nil
-      else {
+      if refs.isEmpty && (!considerExtension || companionRefs.isEmpty) then
+        Nil
+      else
         val nestedCtx = ctx.fresh.addMode(Mode.TypevarsMissContext)
 
         def matchingCandidate(ref: ImplicitRef, extensionOnly: Boolean): Option[Candidate] =
@@ -254,7 +255,6 @@ object Implicits {
         val implicitCandidates =
           refs.flatMap(matchingCandidate(_, extensionOnly = false))
         extensionCandidates ::: implicitCandidates
-      }
     }
   }
 
@@ -1652,6 +1652,8 @@ final class SearchRoot extends SearchHistory {
 sealed class TermRefSet(using Context):
   private val elems = new java.util.LinkedHashMap[TermSymbol, List[Type]]
 
+  def isEmpty = elems.size == 0
+
   def += (ref: TermRef): Unit = {
     val pre = ref.prefix
     val sym = ref.symbol.asTerm

tests/pos/implicit-scope.scala

@@ -9,7 +9,7 @@ object A {
   type FlagSet = opaques.FlagSet
   def FlagSet(bits: Long): FlagSet = opaques.FlagSet(bits)
 
-  extension on (xs: FlagSet) {
+  extension (xs: FlagSet) {
     def bits: Long = opaques.toBits(xs)
     def | (ys: FlagSet): FlagSet = FlagSet(xs.bits | ys.bits)
   }