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: ...

7 files changed, 101 insertions, 3 deletions

diff --git a/test/files/neg/lubs.check b/test/files/neg/lubs.check
index 77ab20102c..affbd4983c 100644
--- a/test/files/neg/lubs.check
+++ b/test/files/neg/lubs.check
@@ -1,5 +1,10 @@
+lubs.scala:10: error: type mismatch;
+ found   : test1.A[test1.A[Object]]
+ required: test1.A[test1.A[test1.A[Any]]]
+  val x3: A[A[A[Any]]] = f
+                         ^
 lubs.scala:11: error: type mismatch;
- found   : test1.A[test1.A[test1.A[Any]]]
+ found   : test1.A[test1.A[Object]]
  required: test1.A[test1.A[test1.A[test1.A[Any]]]]
   val x4: A[A[A[A[Any]]]] = f
                             ^
@@ -13,4 +18,4 @@ lubs.scala:25: error: type mismatch;
  required: test2.A{type T >: Null <: test2.A{type T >: Null <: test2.A{type T >: Null <: test2.A}}}
   val x4: A { type T >: Null <: A { type T >: Null <: A { type T >: Null <: A } } } = f
                                                                                       ^
-three errors found
+four errors found
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)
 }
diff --git a/test/files/run/lub-visibility.check b/test/files/run/lub-visibility.check
index 3461d1bf6b..f3a6bef215 100644
--- a/test/files/run/lub-visibility.check
+++ b/test/files/run/lub-visibility.check
@@ -8,7 +8,7 @@ scala> // should infer List[scala.collection.immutable.Seq[Nothing]]
 scala> // but reverted that for SI-5534.
 
 scala> val x = List(List(), Vector())
-x: List[scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq{def dropRight(n: Int): scala.collection.immutable.Seq[Any] with scala.collection.AbstractSeq[Any]; def takeRight(n: Int): scala.collection.immutable.Seq[Any] with scala.collection.AbstractSeq[Any]; def drop(n: Int): scala.collection.immutable.Seq[Any] with scala.collection.AbstractSeq[Any]; def take(n: Int): scala.collection.immutable.Seq[Any] with scala.collection.AbstractSeq[Any]; def slice(from: Int,until: Int): scala.collection.immutable.Seq[Any] with scala.collection.AbstractSeq[Any]}]; def dropRight(n: Int): scala.collection.immutable.Seq[Nothing] with scala.collection.Ab...
+x: List[scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]{def dropRight(n: Int): scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]; def takeRight(n: Int): scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]; def drop(n: Int): scala.collecti...
 scala> 
 
 scala> 
diff --git a/test/files/run/t2251.check b/test/files/run/t2251.check
new file mode 100644
index 0000000000..55ad2a5857
--- /dev/null
+++ b/test/files/run/t2251.check
@@ -0,0 +1 @@
+Set(List(List(C), Stream(D, ?)))
diff --git a/test/files/run/t2251.scala b/test/files/run/t2251.scala
new file mode 100644
index 0000000000..00c5619b49
--- /dev/null
+++ b/test/files/run/t2251.scala
@@ -0,0 +1,19 @@
+class A
+trait B[T <: B[T]] extends A
+class C extends B[C] { override def toString = "C" }
+class D extends B[D] { override def toString = "D" }
+
+class E {
+  val ys = List(List(new C), Stream(new D))
+}
+
+object Test {
+  def trav = List(List(), Stream())
+
+  def main(args: Array[String]): Unit = {
+    val f = (new E).ys _
+    var xs: Set[List[_ <: Seq[B[_]]]] = Set()
+    xs += f()
+    println(xs)
+  }
+}
diff --git a/test/files/run/t2251b.check b/test/files/run/t2251b.check
new file mode 100644
index 0000000000..42b0be457a
--- /dev/null
+++ b/test/files/run/t2251b.check
@@ -0,0 +1,11 @@
+TypeTag[List[scala.collection.immutable.LinearSeq[B[_ >: D with C <: B[_ >: D with C <: A]]] with scala.collection.AbstractSeq[B[_ >: D with C <: B[_ >: D with C <: A]]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.LinearSeq with scala.collection.AbstractSeq]; def reverse: scala.collection.immutable.LinearSeq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.LinearSeq with scala.collection.AbstractSeq]; def reverse: scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def dropRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]}; def dropRight(n: Int): scala.collection.immutable.LinearSeq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.LinearSeq with scala.collection.AbstractSeq]; def reverse: scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def dropRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]}; def takeRight(n: Int): scala.collection.immutable.LinearSeq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.LinearSeq with scala.collection.AbstractSeq]; def reverse: scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def dropRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]}; def drop(n: Int): scala.collection.immutable.LinearSeq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.LinearSeq with scala.collection.AbstractSeq]; def reverse: scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def dropRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]}; def take(n: Int): scala.collection.immutable.LinearSeq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.LinearSeq with scala.collection.AbstractSeq]; def reverse: scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def dropRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]}; def slice(from: Int,until: Int): scala.collection.immutable.LinearSeq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.LinearSeq with scala.collection.AbstractSeq]; def reverse: scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def dropRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A]}; def splitAt(n: Int): (scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A], scala.collection.immutable.LinearSeq[A] with scala.collection.AbstractSeq[A])}]]
+TypeTag[List[scala.collection.immutable.Iterable[B[_ >: F with E with D with C <: B[_ >: F with E with D with C <: A]]] with F with Int => Any]]
+TypeTag[List[scala.collection.immutable.Seq[B[_ >: D with C <: B[_ >: D with C <: A]]] with scala.collection.AbstractSeq[B[_ >: D with C <: B[_ >: D with C <: A]]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def init: scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]}; def takeRight(n: Int): scala.collection.immutable.Seq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def init: scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]}; def drop(n: Int): scala.collection.immutable.Seq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def init: scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]}; def take(n: Int): scala.collection.immutable.Seq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def init: scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]}; def slice(from: Int,until: Int): scala.collection.immutable.Seq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def init: scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]}; def splitAt(n: Int): (scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A], scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]); def init: scala.collection.immutable.Seq[B[_ >: D with C <: A]] with scala.collection.AbstractSeq[B[_ >: D with C <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def takeRight(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def drop(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def take(n: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def slice(from: Int,until: Int): scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]; def init: scala.collection.immutable.Seq[A] with scala.collection.AbstractSeq[A]}}]]
+TypeTag[List[scala.collection.Set[_ >: G with F <: B[_ >: G with F <: B[_ >: G with F <: A]]]]]
+TypeTag[List[scala.collection.Set[_ >: G with F <: B[_ >: G with F <: B[_ >: G with F <: A]]]]]
+TypeTag[List[scala.collection.Set[_ >: G with F <: B[_ >: G with F <: B[_ >: G with F <: A]]]]]
+TypeTag[List[Seq[B[_ >: G with F <: B[_ >: G with F <: A]]]]]
+TypeTag[List[scala.collection.Map[_ >: F with C <: B[_ >: F with C <: B[_ >: F with C <: A]], B[_ >: G with D <: B[_ >: G with D <: A]]]]]
+TypeTag[List[scala.collection.AbstractSeq[B[_ >: G with F <: B[_ >: G with F <: A]]] with scala.collection.LinearSeq[B[_ >: G with F <: B[_ >: G with F <: A]]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[B[_ >: G with F <: A]] with scala.collection.LinearSeq[B[_ >: G with F <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def drop(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def take(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def slice(from: Int,until: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]}; def drop(n: Int): scala.collection.AbstractSeq[B[_ >: G with F <: A]] with scala.collection.LinearSeq[B[_ >: G with F <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def drop(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def take(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def slice(from: Int,until: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]}; def take(n: Int): scala.collection.AbstractSeq[B[_ >: G with F <: A]] with scala.collection.LinearSeq[B[_ >: G with F <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def drop(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def take(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def slice(from: Int,until: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]}; def slice(from: Int,until: Int): scala.collection.AbstractSeq[B[_ >: G with F <: A]] with scala.collection.LinearSeq[B[_ >: G with F <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def drop(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def take(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def slice(from: Int,until: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]}}]]
+TypeTag[List[Seq[B[_ >: G with F <: B[_ >: G with F <: A]]]]]
+TypeTag[List[Seq[B[_ >: G with F <: B[_ >: G with F <: A]]]]]
diff --git a/test/files/run/t2251b.scala b/test/files/run/t2251b.scala
new file mode 100644
index 0000000000..b67b3aec1e
--- /dev/null
+++ b/test/files/run/t2251b.scala
@@ -0,0 +1,48 @@
+class A
+trait B[T <: B[T]] extends A
+class B1[T <: B1[T]] extends B[T]
+class C extends B[C] { override def toString = "C" }
+class D extends B[D] { override def toString = "D" }
+class E extends B[E] { override def toString = "E" }
+class F extends B[F] { override def toString = "F" }
+class G extends B1[G] { override def toString = "G" }
+
+object Test {
+  import scala.collection.{ mutable, immutable }
+  import scala.collection.immutable.{ Vector }
+  import scala.reflect.runtime.universe._
+  def what[T: TypeTag](x: T) = println(typeTag[T])
+
+  def main(args: Array[String]): Unit = {
+    what(List(List(new C), Stream(new D)))
+    what(List(List(new C), Stream(new D), Vector(new E), Set(new F)))
+    what(List(immutable.Vector(new C), Stream(new D)))
+    what(List(collection.Set(new F), mutable.Set(new G)))
+    what(List(collection.Set(new F), immutable.Set(new G)))
+    what(List(mutable.Set(new F), immutable.Set(new G)))
+    what(List(mutable.Seq(new F), immutable.Seq(new G)))
+    what(List(mutable.Map(new C -> new D), immutable.Map(new F -> new G)))
+    what(List(mutable.MutableList(new F), immutable.List(new G)))
+    what(List(mutable.Seq(new F), collection.Seq(new G)))
+    what(List(mutable.LinearSeq(new F), collection.IndexedSeq(new G)))
+  }
+}
+
+
+// class D extends B[D] { override def toString = "D" }
+
+
+// class E {
+//   val ys = List(List(new C), Stream(new D))
+// }
+
+// object Test {
+//   def trav = List(List(), Stream())
+
+//   def main(args: Array[String]): Unit = {
+//     val f = (new E).ys _
+//     var xs: Set[List[_ <: Seq[B[_]]]] = Set()
+//     xs += f()
+//     println(xs)
+//   }
+// }