Fix desugaring of classes with context bounds

Context bounds did not make it before into secondary
constructors. Now the evidence parameters generated
by context bounds get copied into secondary constructors.

Without this fix, scala.collection.immutable.PagedSeq fails
to compile in new classtag scheme.

Author: odersky

src/dotty/tools/dotc/ast/Desugar.scala

@@ -147,18 +147,7 @@ object desugar {
         tparam
     }
 
-    val meth1 = epbuf.toList match {
-      case Nil =>
-        meth
-      case evidenceParams =>
-        val vparamss1 = vparamss.reverse match {
-          case (vparams @ (vparam :: _)) :: rvparamss if vparam.mods is Implicit =>
-            ((vparams ++ evidenceParams) :: rvparamss).reverse
-          case _ =>
-            vparamss :+ evidenceParams
-        }
-        cpy.DefDef(meth)(tparams = tparams1, vparamss = vparamss1)
-    }
+    val meth1 = addEvidenceParams(cpy.DefDef(meth)(tparams = tparams1), epbuf.toList)
 
     /** The longest prefix of parameter lists in vparamss whose total length does not exceed `n` */
     def takeUpTo(vparamss: List[List[ValDef]], n: Int): List[List[ValDef]] = vparamss match {
@@ -204,6 +193,30 @@ object desugar {
     }
   }
 
+  // Add all evidence parameters in `params` as implicit parameters to `meth` */
+  private def addEvidenceParams(meth: DefDef, params: List[ValDef])(implicit ctx: Context): DefDef =
+    params match {
+      case Nil =>
+        meth
+      case evidenceParams =>
+        val vparamss1 = meth.vparamss.reverse match {
+          case (vparams @ (vparam :: _)) :: rvparamss if vparam.mods is Implicit =>
+            ((vparams ++ evidenceParams) :: rvparamss).reverse
+          case _ =>
+            meth.vparamss :+ evidenceParams
+        }
+        cpy.DefDef(meth)(vparamss = vparamss1)
+    }
+
+  /** The implicit evidence parameters of `meth`, as generated by `desugar.defDef` */
+  private def evidenceParams(meth: DefDef)(implicit ctx: Context): List[ValDef] =
+    meth.vparamss.reverse match {
+      case (vparams @ (vparam :: _)) :: _ if vparam.mods is Implicit =>
+        vparams.dropWhile(!_.name.startsWith(nme.EVIDENCE_PARAM_PREFIX))
+      case _ =>
+        Nil
+    }
+
   /** Fill in empty type bounds with Nothing/Any. Expand private local type parameters as follows:
    *
    *     class C[v T]
@@ -255,10 +268,13 @@ object desugar {
       else constr1.vparamss.nestedMap(toDefParam)
     val constr = cpy.DefDef(constr1)(tparams = constrTparams, vparamss = constrVparamss)
 
-    // Add constructor type parameters to auxiliary constructors
+    // Add constructor type parameters and evidence implicit parameters
+    // to auxiliary constructors
     val normalizedBody = impl.body map {
       case ddef: DefDef if ddef.name.isConstructorName =>
-        cpy.DefDef(ddef)(tparams = constrTparams)
+        addEvidenceParams(
+          cpy.DefDef(ddef)(tparams = constrTparams),
+          evidenceParams(constr1).map(toDefParam))
       case stat =>
         stat
     }