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, 11 insertions, 0 deletions

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]]]]]