SI-6651 Substitute `this` in extension method sigs

This allows for the likes of:

    class A[X](val x: X) extends AnyVal {
      def foo(xy: x.Y) {}
    }

We have to do this in both directions, when synthesizing
the extension method in `Extender#transform`, and later on
when Erasure tries to find the corresponding extension
methods by backing out the original signatures from
the signatures of the synthesized methods in the companion.

In the first case, we have to be careful to use a stable
reference to the `self` parameter, which can satisfy the
dependent types.

1 files changed, 30 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/ExtensionMethods.scala b/src/compiler/scala/tools/nsc/transform/ExtensionMethods.scala
index 39e16c3f58..589aa43ac2 100644
--- a/src/compiler/scala/tools/nsc/transform/ExtensionMethods.scala
+++ b/src/compiler/scala/tools/nsc/transform/ExtensionMethods.scala
@@ -75,7 +75,19 @@ abstract class ExtensionMethods extends Transform with TypingTransformers {
     val candidates = extensionNames(imeth) map (companionInfo.decl(_)) filter (_.exists)
     val matching = candidates filter (alt => normalize(alt.tpe, imeth.owner) matches imeth.tpe)
     assert(matching.nonEmpty,
-      s"no extension method found for $imeth:${imeth.tpe} among ${candidates.map(c => c.name+":"+c.tpe).toList} / ${extensionNames(imeth).toList}")
+      sm"""|no extension method found for:
+           |
+           |  $imeth:${imeth.tpe}
+           |
+           | Candidates:
+           |
+           | ${candidates.map(c => c.name+":"+c.tpe).mkString("\n")}
+           |
+           | Candidates (signatures normalized):
+           |
+           | ${candidates.map(c => c.name+":"+normalize(c.tpe, imeth.owner)).mkString("\n")}
+           |
+           | Eligible Names: ${extensionNames(imeth).mkString(",")}"""")
     matching.head
   }
 
@@ -94,11 +106,18 @@ abstract class ExtensionMethods extends Transform with TypingTransformers {
    */
   private def normalize(stpe: Type, clazz: Symbol): Type = stpe match {
     case PolyType(tparams, restpe) =>
-      GenPolyType(tparams dropRight clazz.typeParams.length, normalize(restpe.substSym(tparams takeRight clazz.typeParams.length, clazz.typeParams), clazz))
+      // Split the type parameters of the extension method into two groups,
+      // corresponding the to class and method type parameters.
+      val numClassParams = clazz.typeParams.length
+      val methTParams = tparams dropRight numClassParams
+      val classTParams = tparams takeRight numClassParams
+
+      GenPolyType(methTParams,
+        normalize(restpe.substSym(classTParams, clazz.typeParams), clazz))
     case MethodType(List(thiz), restpe) if thiz.name == nme.SELF =>
-      restpe
-    case MethodType(tparams, restpe) =>
-      MethodType(tparams.drop(1), restpe)
+      restpe.substituteTypes(thiz :: Nil, clazz.thisType :: Nil)
+    case MethodType(thiz :: params, restpe) =>
+      MethodType(params, restpe)
     case _ =>
       stpe
   }
@@ -128,7 +147,11 @@ abstract class ExtensionMethods extends Transform with TypingTransformers {
           MethodType(List(thisParam), restpe)
       }
       val GenPolyType(tparams, restpe) = origInfo cloneInfo extensionMeth
-      GenPolyType(tparams ::: newTypeParams, transform(restpe) substSym (clazz.typeParams, newTypeParams))
+      val selfParamSingletonType = singleType(currentOwner.companionModule.thisType, thisParam)
+      GenPolyType(
+        tparams ::: newTypeParams,
+        transform(restpe) substThisAndSym (clazz, selfParamSingletonType, clazz.typeParams, newTypeParams)
+      )
     }
 
     private def allParams(tpe: Type): List[Symbol] = tpe match {
@@ -162,7 +185,7 @@ abstract class ExtensionMethods extends Transform with TypingTransformers {
             origMeth.defString,
             extensionMeth.defString)) // extensionMeth.defStringSeenAs(origInfo
 
-          def thisParamRef = gen.mkAttributedIdent(extensionMeth.info.params.head setPos extensionMeth.pos)
+          def thisParamRef = gen.mkAttributedStableRef(extensionMeth.info.params.head setPos extensionMeth.pos)
           val GenPolyType(extensionTpeParams, extensionMono) = extensionMeth.info
           val origTpeParams = (tparams map (_.symbol)) ::: currentOwner.typeParams
           val extensionBody = rhs