Simpler cycle detection

Turns out that the last commit was a red herring. None of the hoops
it jumped though was necessary. Instead there was a bug in isRef
which caused `&` to erroneously compute T & Int as Int.

The bug was that we always approximated alias types by their high bound. But
in the present case, this leads to errors because U gets 'bounds >: Nothing <: Any',
but it was still an alias type (i.e. its Deferred flag is not set). The fix
dereferences aliases only if their info is a TypeAlias.

1 files changed, 5 insertions, 0 deletions

diff --git a/tests/pos/prefix.scala b/tests/pos/prefix.scala
new file mode 100644
index 000000000..e18eaa50f
--- /dev/null
+++ b/tests/pos/prefix.scala
@@ -0,0 +1,5 @@
+
+abstract class ClassPath {
+  type AnyClassRep = ClassPath#ClassRep
+  type ClassRep
+}