Make Predef capture checked (#24643)

This was forgotten before when we added capture-checked checked stdlib.
Maybe it was impossible before due to bootstrap problems? But now it is
possible.
[Cherry-picked 8067622]

Author: odersky

library/src/scala/Array.scala

@@ -77,9 +77,9 @@ object Array {
     case _ => it.iterator.toArray[A]
   }
 
-  private def slowcopy(src : AnyRef,
+  private def slowcopy(src : AnyRef^,
                        srcPos : Int,
-                       dest : AnyRef,
+                       dest : AnyRef^,
                        destPos : Int,
                        length : Int): Unit = {
     var i = srcPos
@@ -107,7 +107,7 @@ object Array {
    *
    *  @see `java.lang.System#arraycopy`
    */
-  def copy(src: AnyRef, srcPos: Int, dest: AnyRef, destPos: Int, length: Int): Unit = {
+  def copy(src: AnyRef^, srcPos: Int, dest: AnyRef^, destPos: Int, length: Int): Unit = {
     val srcClass = src.getClass
     val destClass = dest.getClass
     if (srcClass.isArray && ((destClass eq srcClass) ||

library/src/scala/Predef.scala

@@ -13,8 +13,8 @@
 package scala
 
 import scala.language.`2.13`
+import language.experimental.captureChecking
 import scala.language.implicitConversions
-
 import scala.collection.{mutable, immutable, ArrayOps, StringOps}, immutable.WrappedString
 import scala.annotation.{elidable, experimental, implicitNotFound, publicInBinary, targetName }, elidable.ASSERTION
 import scala.annotation.meta.{ companionClass, companionMethod }
@@ -564,7 +564,7 @@ object Predef extends LowPriorityImplicits {
    *  `(A => A) <: (A => B)`.
    */
   // $ to avoid accidental shadowing (e.g. scala/bug#7788)
-  implicit def $conforms[A]: A => A = <:<.refl
+  implicit def $conforms[A]: A -> A = <:<.refl
 
   // Extension methods for working with explicit nulls