Remove the helpers in scala.deriving.

* `ArrayProduct`
* `EmptyProduct`
* `productElement`

These helpers are not mandated by the spec, nor used by the
compiler. They were used in a few tests for typeclass derivation,
but it does not seem that they are fundamental. We define them in
the relevant tests instead.

`ArrayProduct` and `EmptyProduct` seem to be used by "unpickling"
kinds of derivations. This is a fairly specific use case, and
unpicklers can and should define their own.

`productElement` seems to be more generally useful, but at the same
time is very unsafe, and is not hard to re-implement. As it was the
last non-fundamental definition remaining in `scala.deriving`, we
also remove it. It could be reintroduced in the future if more
widespread experience proves that it is widely useful.

Author: sjrd

library/src/scala/deriving/Helpers.scala

@@ -19,6 +19,9 @@ object JsonEncoder {
       case _ => Nil
     }
 
+  private def productElement[T](x: Any, idx: Int): T =
+    x.asInstanceOf[Product].productElement(idx).asInstanceOf[T]
+
   inline def derived[T](implicit ev: Mirror.Of[T]): JsonEncoder[T] = new JsonEncoder[T] {
     def encode(value: T): String =
       inline ev match {
@@ -45,4 +48,4 @@ object JsonEncoder {
   given stringEncoder as JsonEncoder[String] {
     def encode(value: String) = value
   }
-}
+}

tests/disabled/pos-macros/i7853/JsonEncoder_1.scala

@@ -213,6 +213,9 @@ object typeclasses {
     def eql(x: T, y: T): Boolean
   }
 
+  private def productElement[T](x: Any, idx: Int): T =
+    x.asInstanceOf[Product].productElement(idx).asInstanceOf[T]
+
   object Eq {
     import scala.compiletime.erasedValue
     import compiletime._
@@ -273,6 +276,17 @@ object typeclasses {
 
     def nextInt(buf: mutable.ListBuffer[Int]): Int = try buf.head finally buf.trimStart(1)
 
+    sealed class ArrayProduct(val elems: Array[AnyRef]) extends Product {
+      def this(size: Int) = this(new Array[AnyRef](size))
+      def canEqual(that: Any): Boolean = true
+      def productElement(n: Int): Any = elems(n)
+      def productArity: Int = elems.length
+      override def productIterator: Iterator[Any] = elems.iterator
+      def update(n: Int, x: Any): Unit = elems(n) = x.asInstanceOf[AnyRef]
+    }
+
+    object EmptyProduct extends ArrayProduct(Array.emptyObjectArray)
+
     inline def tryPickle[T](buf: mutable.ListBuffer[Int], x: T): Unit = summonInline[Pickler[T]].pickle(buf, x)
 
     inline def pickleElems[Elems <: Tuple](n: Int)(buf: mutable.ListBuffer[Int], x: Any): Unit =
@@ -391,4 +405,4 @@ object Test extends App {
   implicitly[Pickler[E14[Int]]]
   implicitly[Pickler[E15[Int]]]
   implicitly[Pickler[E16[Int]]]
-}
+}

tests/pos-special/typeclass-scaling.scala

@@ -8,7 +8,7 @@ sealed trait U
 case class C() extends U
 
 object Test extends App {
-  import deriving.{Mirror, EmptyProduct}
+  import deriving._
 
   case class AA[X >: Null <: AnyRef](x: X, y: X, z: String)
 
@@ -21,7 +21,7 @@ object Test extends App {
   }
   summon[Mirror.Of[B.type]] match {
     case m: Mirror.Product =>
-      println(m.fromProduct(EmptyProduct))
+      println(m.fromProduct(EmptyTuple))
   }
   summon[Mirror.Of[T]] match {
     case m: Mirror.SumOf[T] =>

tests/run/deriving.scala

@@ -30,6 +30,9 @@ object typeclasses {
     def eql(x: T, y: T): Boolean
   }
 
+  private def productElement[T](x: Any, idx: Int): T =
+    x.asInstanceOf[Product].productElement(idx).asInstanceOf[T]
+
   object Eq {
     import scala.compiletime.{erasedValue, summonFrom}
     import compiletime._
@@ -92,6 +95,17 @@ object typeclasses {
 
     def nextInt(buf: mutable.ListBuffer[Int]): Int = try buf.head finally buf.trimStart(1)
 
+    sealed class ArrayProduct(val elems: Array[AnyRef]) extends Product {
+      def this(size: Int) = this(new Array[AnyRef](size))
+      def canEqual(that: Any): Boolean = true
+      def productElement(n: Int): Any = elems(n)
+      def productArity: Int = elems.length
+      override def productIterator: Iterator[Any] = elems.iterator
+      def update(n: Int, x: Any): Unit = elems(n) = x.asInstanceOf[AnyRef]
+    }
+
+    object EmptyProduct extends ArrayProduct(Array.emptyObjectArray)
+
     inline def tryPickle[T](buf: mutable.ListBuffer[Int], x: T): Unit = summonFrom {
       case pkl: Pickler[T] => pkl.pickle(buf, x)
     }