SI-8276 Test for cyclic error caused by (reverted) SI-1786 fix

We reverted SI-1786 recently on the grounds that its means of
avoiding cycles (not sharpening bounds of T[_] when T is
uninitialized) caused unacceptable non-determinism (well:
compilation order dependency) to type inference.

Nary a day later, @gkossakowski hit a regression in scala-io.
Bisection revealed that it stopped working in 2dbd17a2 and
started working agiain after the revert. How's that for
prescience!

I've distilled the cyclic error in scala-io in this test
case.

I'm yet to pinpoint this, followon error, which didn't survive
the shrink ray, and only appeared in the original code:

    error: java.lang.IndexOutOfBoundsException: 0
	at scala.collection.LinearSeqOptimized$class.apply(LinearSeqOptimized.scala:51)
	at scala.collection.immutable.List.apply(List.scala:83)
	at scala.reflect.internal.tpe.TypeMaps$AsSeenFromMap.correspondingTypeArgument(TypeMaps.scala:5

1 files changed, 57 insertions, 0 deletions

diff --git a/test/files/pos/t1786-cycle.scala b/test/files/pos/t1786-cycle.scala
new file mode 100644
index 0000000000..af5d892c6a
--- /dev/null
+++ b/test/files/pos/t1786-cycle.scala
@@ -0,0 +1,57 @@
+trait GenTraversableLike[+A, +Repr] extends Any
+
+object O {
+  (null: Any) match {
+    case _: LongTraversableLike[_] =>
+  }
+}
+
+trait LongTraversable extends LongTraversableLike[LongTraversable]
+
+trait LongTraversableLike[+Repr <: LongTraversableLike[Repr]] extends GenTraversableLike[Any, Repr]
+
+/*
+% scalac-hash v2.11.0-M8 test/files/pos/t1786-cycle.scala
+[warn] v2.11.0-M8 failed, using closest available
+test/files/pos/t1786-cycle.scala:11: error: illegal cyclic reference involving trait LongTraversableLike
+trait LongTraversableLike[+Repr <: LongTraversableLike[Repr]] extends GenTraversableLike[Any, Repr]
+                                                                      ^
+one error found
+
+Okay again after SI-1786 was reverted.
+
+
+|-- object O BYVALmode-EXPRmode (site: package <empty>)
+|    |-- super EXPRmode-POLYmode-QUALmode (silent: <init> in O)
+|    |    |-- this EXPRmode (silent: <init> in O)
+|    |    |    \-> O.type
+|    |    \-> O.type
+|    |-- (null: Any) match { case (_: LongTraversableLike[(_ @ <em... BYVALmode-EXPRmode (site: value <local O> in O)
+|    |    |-- (null: Any) BYVALmode-EXPRmode (site: value <local O> in O)
+|    |    |    |-- Any TYPEmode (site: value <local O> in O)
+|    |    |    |    \-> Any
+|    |    |    |-- null : pt=Any EXPRmode (site: value <local O> in O)
+|    |    |    |    \-> Null(null)
+|    |    |    \-> Any
+|    |    |-- (_: LongTraversableLike[(_ @ <empty>)]) : pt=Any PATTERNmode (site: value <local O> in O) enrichment only
+|    |    |    |-- LongTraversableLike[(_ @ <empty>)] TYPEPATmode-TYPEmode (site: value <local O> in O) enrichment only
+|    |    |    |    |--  <: LongTraversableLike[Repr] TYPEmode (site: type Repr in <empty>)
+|    |    |    |    |    |-- LongTraversableLike[Repr] TYPEmode (site: type Repr in <empty>)
+|    |    |    |    |    |    |-- Repr NOmode (site: type Repr in <empty>)
+|    |    |    |    |    |    |    \-> Repr
+|    |    |    |    |    |    \-> LongTraversableLike[Repr]
+|    |    |    |    |    [adapt]  <: LongTraversableLike[Repr] is now a TypeTree( <: LongTraversableLike[Repr])
+|    |    |    |    |    \->  <: LongTraversableLike[Repr]
+|    |    |    |    |-- (_ @ <empty>) TYPEPATmode-TYPEmode (site: value <local O> in O) enrichment only
+|    |    |    |    |    \-> _
+|    |    |    |    |-- GenTraversableLike FUNmode-TYPEmode (site: trait LongTraversableLike)
+|    |    |    |    |    \-> GenTraversableLike
+|    |    |    |    |-- GenTraversableLike[Any, Repr] TYPEmode (site: trait LongTraversableLike)
+|    |    |    |    |    |-- Any TYPEmode (site: trait LongTraversableLike)
+|    |    |    |    |    |    \-> Any
+|    |    |    |    |    |-- Repr TYPEmode (site: trait LongTraversableLike)
+|    |    |    |    |    |    \-> Repr
+|    |    |    |    |    caught scala.reflect.internal.Symbols$CyclicReference: illegal cyclic reference involving trait LongTraversableLike: while typing GenTraversableLike[Any, Repr]
+test/files/pos/t1786-cycle.scala:11: error: illegal cyclic reference involving trait LongTraversableLike
+trait LongTraversableLike[+Repr <: LongTraversableLike[Repr]] extends GenT
+*/
\ No newline at end of file