Avoid comparing unkown atoms of non-singleton types

Author: mbovel

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

@@ -1589,7 +1589,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
               Some(verified(false))
             else
               None
-          case _ => Some(verified(recur(tp1, NothingType)))
+          case Atoms.Unknown =>
+            if knownSingletons then
+              Some(verified(recur(tp1, NothingType)))
+            else
+              None
       case _ => None
 
   /** Subtype test for corresponding arguments in `args1`, `args2` according to

tests/pos/16583.scala

@@ -0,0 +1,14 @@
+import compiletime.constValueTuple
+
+val ll0: Tuple3["one", "two", "three"] = constValueTuple[("one", "two", "three")]
+val ll1 = constValueTuple[("one", "two", "three")].toList
+val ll3: List["one" | ("two" | ("three" | Nothing))] = constValueTuple[("one", "two", "three")].toList
+val ll4: List["one" | ("two" | "three")] = constValueTuple[("one", "two", "three")].toList
+
+inline def labels[Labels <: Tuple](using ev: Tuple.Union[Labels] <:< String): List[String] =
+  val tmp = constValueTuple[Labels].toList
+  ev.substituteCo( tmp )
+
+def test = labels[("one", "two", "three")].mkString("<", ", ", ">")
+
+def test2(tup: ("one", "two", "three")) = tup.toList