Avoid infinite recursion when comparing Nothing and union types

Author: odersky

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

@@ -601,7 +601,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] w
       case OrType(tp21, tp22) =>
         if (tp2.atoms.nonEmpty && canCompare(tp2.atoms))
           return tp1.atoms.nonEmpty && tp1.atoms.subsetOf(tp2.atoms) ||
-            tp1.isRef(NothingClass)
+            tp1.widen.isRef(NothingClass)
 
         // The next clause handles a situation like the one encountered in i2745.scala.
         // We have:

tests/pos/singlesubtypes.scala

@@ -0,0 +1,12 @@
+object E {
+  val a: String = ???
+  val b: String = ???
+}
+
+object Test {
+
+  val a: E.a.type = E.a
+  val b: E.b.type = E.b
+
+  val c: a.type | b.type = ???
+}