Improvement for SI-2251, failure to lub f-bounds.

After a great struggle, I realized that the major reason that
code like this still doesn't compile:

  List(Stream(), List())

is that we were poisoning the computed lub in mergePrefixAndArgs by
throwing in Any when the max recursion depth was reached. I modified
it to return NoType instead, which allowed me to teach lublist to
recognize what has happened and fall back to a weaker type, one
which does not contain recursive bounds. This enables the lubbing
process to complete. The most elusive lub, defeated. Notice also
that the refinement members are correctly parameterized on Nothing,
rather than on Any as has often been the case.

scala> List(Stream(), List())
res0: List[scala.collection.immutable.LinearSeq[Nothing] with scala.collection.AbstractSeq[Nothing]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.LinearSeq with scala.collection.AbstractSeq]; def reverse: scala.collection.immutable.LinearSeq[Nothing] with scala.collection.AbstractSeq[Nothing]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.LinearSeq with scala.collection.AbstractSeq]; def reverse: scala.collection.immutable.LinearSeq[Nothing] with scala.collection.AbstractSeq[Nothing]{def reverse: scala.collection.immutable.LinearSeq[Nothing] with scala.collection.AbstractSeq[Nothing]; def dropRight(n: Int): scala.collection.immutable.LinearSeq[Nothing] with scala.collection.AbstractSeq[Nothing]; def takeRight(n: ...

1 files changed, 14 insertions, 0 deletions

diff --git a/test/files/pos/ticket2251.scala b/test/files/pos/ticket2251.scala
index b3afee4ea9..c220e85350 100644
--- a/test/files/pos/ticket2251.scala
+++ b/test/files/pos/ticket2251.scala
@@ -22,4 +22,18 @@ lub of List(D, C) is B[_2] forSome { type _2 >: D with C{} <: B[_1] forSome { ty
   // should be: B[X] forSome {type X <: B[X]} -- can this be done automatically? for now, just detect f-bounded polymorphism and fall back to more coarse approximation
 
   val data: List[A] = List(new C, new D)
+
+  val data2 = List(new C, new D)
+
+  val data3: List[B[X] forSome { type X <: B[_ <: A] }] = List(new C, new D)
+
+  // Not yet --
+  // val data4: List[B[X] forSome { type X <: B[X] }] = List(new C, new D)
+  // <console>:7: error: type mismatch;
+  //  found   : List[B[_ >: D with C <: B[_ >: D with C <: A]]]
+  //  required: List[B[X] forSome { type X <: B[X] }]
+  //        val data4: List[B[X] forSome { type X <: B[X] }] = List(new C, new D)
+
+  // works
+  val data5 = List[B[X] forSome { type X <: B[X] }](new C, new D)
 }