Handle SingleDenotations in ExtensionMethods

The missing case was discovered while experimenting with different opaque
type encodings. It has nothing to do with opaque types, just with the fact
that we sometimes got a UniqueRefDenotation instead of a SymDenotation. But
this should not make a difference.

Author: odersky

compiler/src/dotty/tools/dotc/transform/ExtensionMethods.scala

@@ -113,7 +113,16 @@ class ExtensionMethods extends MiniPhase with DenotTransformer with FullParamete
       }
       ref1
     case _ =>
-      ref
+      ref.info match {
+        case ClassInfo(pre, cls, _, _, _) if cls is ModuleClass =>
+          cls.linkedClass match {
+            case valueClass: ClassSymbol if isDerivedValueClass(valueClass) =>
+              val info1 = cls.denot(ctx.withPhase(ctx.phase.next)).asClass.classInfo.derivedClassInfo(prefix = pre)
+              ref.derivedSingleDenotation(ref.symbol, info1)
+            case _ => ref
+          }
+        case _ => ref
+      }
   }
 
   protected def rewiredTarget(target: Symbol, derived: Symbol)(implicit ctx: Context): Symbol =
@@ -210,13 +219,14 @@ object ExtensionMethods {
   def extensionMethod(imeth: Symbol)(implicit ctx: Context): TermSymbol =
     ctx.atPhase(ctx.extensionMethodsPhase.next) { implicit ctx =>
       // FIXME use toStatic instead?
-      val companionInfo = imeth.owner.companionModule.info
+      val companion = imeth.owner.companionModule
+      val companionInfo = companion.info
       val candidates = extensionNames(imeth) map (companionInfo.decl(_).symbol) filter (_.exists)
       val matching = candidates filter (c => FullParameterization.memberSignature(c.info) == imeth.signature)
       assert(matching.nonEmpty,
        i"""no extension method found for:
           |
-          |  $imeth:${imeth.info.show} with signature ${imeth.signature}
+          |  $imeth:${imeth.info.show} with signature ${imeth.signature} in ${companion.moduleClass}
           |
           | Candidates:
           |