Never fail a subtype test if atoms don't compare

Singleton types can be aliases of other singleton types (or unions
of singletons) in complex ways which atoms don't reflect currently.
So to be conservative we should use atoms only for confirming
subtype relationships, never for rejecting them.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -432,9 +432,8 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] w
           recur(tp1.widenSingletons, tp2)
 
         if (tp2.atoms.nonEmpty && canCompare(tp2.atoms))
-          tp1.atoms.nonEmpty && tp1.atoms.subsetOf(tp2.atoms)
-        else
-          widenOK || joinOK || recur(tp11, tp2) && recur(tp12, tp2)
+          if (tp1.atoms.nonEmpty && tp1.atoms.subsetOf(tp2.atoms)) return true
+        widenOK || joinOK || recur(tp11, tp2) && recur(tp12, tp2)
       case tp1: MatchType =>
         val reduced = tp1.reduced
         if (reduced.exists) recur(reduced, tp2) else thirdTry
@@ -600,8 +599,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] w
         compareTypeLambda
       case OrType(tp21, tp22) =>
         if (tp2.atoms.nonEmpty && canCompare(tp2.atoms))
-          return tp1.atoms.nonEmpty && tp1.atoms.subsetOf(tp2.atoms) ||
-            tp1.widen.isRef(NothingClass)
+          if (tp1.atoms.nonEmpty && tp1.atoms.subsetOf(tp2.atoms)) return true
 
         // The next clause handles a situation like the one encountered in i2745.scala.
         // We have:

tests/pos/singlesubtypes.scala

@@ -9,4 +9,5 @@ object Test {
   val b: E.b.type = E.b
 
   val c: a.type | b.type = ???
+  val d: a.type | b.type = c
 }