Added test to TreeChecker that guards against orphan parameters.

Currently, tests fail. The failures I checked are all related to tailcalls.
Not sure whether there are others.

This is a blocker for serialization. Orphan parameters cannot be serialized.

Maybe rethink the position of tailcalls? It looks to me that the repeated trouble it gives us is
more than the effort required to put an efficient tailcall recognition after pattern matching in place.
But I might be wrong.

Author: odersky

src/dotty/tools/dotc/Compiler.scala

@@ -41,7 +41,7 @@ class Compiler {
       List(new FirstTransform,
            new SyntheticMethods),
       List(new SuperAccessors),
-      // pickling goes here
+      //List(new Pickler), // Pickler needs to come last in a group since it should not pickle trees generated later
       List(new RefChecks,
            new ElimRepeated,
            new ElimLocals,

src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -139,9 +139,28 @@ class TreeChecker {
             assert(isSubType(tree1.tpe, tree.typeOpt), divergenceMsg(tree1.tpe, tree.typeOpt))
           tree1
       }
+      checkNoOrphans(res.tpe)
       phasesToCheck.foreach(_.checkPostCondition(res))
       res
     }
+    
+    /** Check that PolyParams and MethodParams refer to an enclosing type */
+    def checkNoOrphans(tp: Type)(implicit ctx: Context) = new TypeMap() {
+      val definedBinders = mutable.Set[Type]()
+      def apply(tp: Type): Type = {
+        tp match {
+          case tp: BindingType => 
+            definedBinders += tp
+            mapOver(tp)
+            definedBinders -= tp
+          case tp: ParamType =>
+            assert(definedBinders.contains(tp.binder), s"orphan param: $tp")
+          case _ =>
+            mapOver(tp)
+        }
+        tp
+      }
+    }.apply(tp)
 
     override def typedIdent(tree: untpd.Ident, pt: Type)(implicit ctx: Context): Tree = {
       assert(tree.isTerm || !ctx.isAfterTyper, tree.show + " at " + ctx.phase)

test/dotc/tests.scala

@@ -15,7 +15,7 @@ class tests extends CompilerTest {
 
   implicit val defaultOptions = noCheckOptions ++ List(
       "-Yno-deep-subtypes",
-      "-Ycheck:resolveSuper,mixin,restoreScopes",
+      "-Ycheck:tailrec,resolveSuper,mixin,restoreScopes",
       "-d", "./out/"
   )