optimization: avoid NotGiven allocations

Author: fwbrasil

library/src/scala/util/NotGiven.scala

@@ -35,7 +35,7 @@ object NotGiven extends LowPriorityNotGiven {
    *  search for `C`). A reference to this value will be explicitly constructed by Dotty's
    *  implicit search algorithm
    */
-  def value: NotGiven[Nothing] = new NotGiven[Nothing]()
+  val value: NotGiven[Nothing] = new NotGiven[Nothing]()
 
   /** One of two ambiguous methods used to emulate negation in Scala 2 */
   given amb1[T](using ev: T): NotGiven[T] = ???