Fix extension methods with unused parameters

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -196,7 +196,8 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
       case tp: MethodType =>
         def valueParam(name: TermName, info: Type): TermSymbol = {
           val maybeImplicit = if (tp.isImplicitMethod) Implicit else EmptyFlags
-          ctx.newSymbol(sym, name, TermParam | maybeImplicit, info)
+          val maybeUnused = if (tp.isUnusedMethod) Unused else EmptyFlags
+          ctx.newSymbol(sym, name, TermParam | maybeImplicit | maybeUnused, info)
         }
         val params = (tp.paramNames, tp.paramInfos).zipped.map(valueParam)
         val (paramss, rtp) = valueParamss(tp.instantiate(params map (_.termRef)))

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

@@ -229,14 +229,17 @@ trait FullParameterization {
         .appliedToTypes(allInstanceTypeParams(originalDef, abstractOverClass).map(_.typeRef))
         .appliedTo(This(originalDef.symbol.enclosingClass.asClass))
 
+    def refOrDefault(tree: Tree): Tree = // use deafult values for
+      if (tree.symbol is Flags.Unused) tpd.defaultValue(tree.tpe) else ref(tree.symbol)
+
     (if (!liftThisType)
-      fun.appliedToArgss(originalDef.vparamss.nestedMap(vparam => ref(vparam.symbol)))
+      fun.appliedToArgss(originalDef.vparamss.nestedMap(vparam => refOrDefault(vparam)))
     else {
       // this type could have changed on forwarding. Need to insert a cast.
       val args = (originalDef.vparamss, fun.tpe.paramInfoss).zipped.map((vparams, paramTypes) =>
         (vparams, paramTypes).zipped.map((vparam, paramType) => {
           assert(vparam.tpe <:< paramType.widen) // type should still conform to widened type
-          ref(vparam.symbol).ensureConforms(paramType)
+          refOrDefault(vparam).ensureConforms(paramType)
         })
       )
       fun.appliedToArgss(args)

tests/pos/unused-extension-method.scala

@@ -0,0 +1,3 @@
+class IntDeco(x: Int) extends AnyVal {
+  def foo(unused y: Int) = x
+}