SI-7126 Account for the alias types that don't dealias.

After this change:

    qbin/scalac -Ydebug test/files/pos/t7126.scala 2>&1 | grep warning
    warning: dropExistential did not progress dealiasing Test.this.T[Test.this.T], see SI-7126
    one warning found

T[T]? Really? The true bug lies somewhere else; the comments of
the ticket illuminate the general areas of concern.

1 files changed, 11 insertions, 0 deletions

diff --git a/test/files/pos/t7126.scala b/test/files/pos/t7126.scala
new file mode 100644
index 0000000000..6720511e08
--- /dev/null
+++ b/test/files/pos/t7126.scala
@@ -0,0 +1,11 @@
+import language._
+
+object Test {
+  type T = Any
+  boom(???): Option[T] // SOE
+  def boom[CC[U]](t : CC[T]): Option[CC[T]] = None
+
+  // okay
+  foo(???): Option[Any]
+  def foo[CC[U]](t : CC[Any]): Option[CC[Any]] = None
+}
\ No newline at end of file