Fix checking whether types are instantiable.

The logic for checking aginst the self type was wrong, as demonstrated
by pos/checkInstantiable.scala.

Author: odersky

src/dotty/tools/dotc/typer/Checking.scala

@@ -49,8 +49,11 @@ object Checking {
         if (cls.is(AbstractOrTrait))
           ctx.error(d"$cls is abstract; cannot be instantiated", pos)
         if (!cls.is(Module)) {
-          val selfType = tp.givenSelfType.asSeenFrom(tref.prefix, cls.owner)
-          if (selfType.exists && !(tp <:< selfType))
+          // Create a synthetic singleton type instance, and check whether
+          // it conforms to the self type of the class as seen from that instance.
+          val stp = SkolemType(tp)
+          val selfType = tref.givenSelfType.asSeenFrom(stp, cls)
+          if (selfType.exists && !(stp <:< selfType))
             ctx.error(d"$tp does not conform to its self type $selfType; cannot be instantiated")
         }
       case _ =>

tests/pos/checkInstantiable.scala

@@ -0,0 +1,5 @@
+// check instantiable of parameterized self type
+class LS[T] { self: LS[T] => }
+object Test {
+  new LS[Int]
+}