Fix Singleton handling of compile-time operations

Author: dwijnand

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

@@ -1504,7 +1504,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         if (fromBelow) recur(other, folded) else recur(folded, other)
       } || other.match {
         case other: TypeRef if !fromBelow && other.symbol == defn.SingletonClass =>
-          true // Even if not declared, the result of a compiletime operator is a constant type
+          tp.args.forall(arg => isSubType(arg, defn.SingletonType))
+          // Compile-time operations with singleton arguments are singletons
         case _ => false
       }
     }