Reorder unpickled type params if necessary

When compiling Iterator.scala it was observed that
the type parameters of BufferedCanBuildFrom appeared
inm the wrong order. This fix corrects that, making
sure that type parameters appear in the decls scope
in the same order as they are given in the epxlicitly
unpickled type parameter list.

Author: odersky

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -176,7 +176,7 @@ object SymDenotations {
       // completions.println(s"completed ${this.debugString}")
     }
 
-    protected[dotc] final def info_=(tp: Type) = {
+    protected[dotc] def info_=(tp: Type) = {
       /* // DEBUG
        def illegal: String = s"illegal type for $this: $tp"
       if (this is Module) // make sure module invariants that allow moduleClass and sourceModule to work are kept.
@@ -1177,6 +1177,11 @@ object SymDenotations {
       myTypeParams
     }
 
+    override protected[dotc] final def info_=(tp: Type) = {
+      super.info_=(tp)
+      myTypeParams = null // changing the info might change decls, and with it typeParams
+    }
+
     /** The denotations of all parents in this class. */
     def classParents(implicit ctx: Context): List[TypeRef] = info match {
       case classInfo: ClassInfo => classInfo.classParents

src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -130,8 +130,18 @@ object Scala2Unpickler {
     } else {
       registerCompanionPair(scalacCompanion, denot.classSymbol)
     }
+    val declsTypeParams = denot.typeParams
+    val declsInRightOrder =
+      if (declsTypeParams.corresponds(tparams)(_.name == _.name)) decls
+      else { // create new scope with type parameters in right order
+        val decls1 = newScope
+        for (tparam <- tparams) decls1.enter(decls.lookup(tparam.name))
+        for (sym <- decls) if (!declsTypeParams.contains(sym)) decls1.enter(sym)
+        decls1
+      }
 
-    denot.info = ClassInfo(denot.owner.thisType, denot.classSymbol, parentRefs, decls, ost)
+    denot.info = ClassInfo(
+      denot.owner.thisType, denot.classSymbol, parentRefs, declsInRightOrder, ost)
   }
 }