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, 4 insertions, 3 deletions

diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 33d108540..feee05d86 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -104,9 +104,10 @@ object Types {
      */
     def isRef(sym: Symbol)(implicit ctx: Context): Boolean = stripTypeVar match {
       case this1: TypeRef =>
-        val thissym = this1.symbol
-        if (thissym.isAliasType) this1.info.bounds.hi.isRef(sym)
-        else thissym eq sym
+        this1.info match { // see comment in Namers/typeDefSig
+          case TypeBounds(lo, hi) if lo eq hi => hi.isRef(sym)
+          case _ =>  this1.symbol eq sym
+        }
       case this1: RefinedType =>
         // make sure all refinements are type arguments
         this1.parent.isRef(sym) && this.argInfos.nonEmpty