[backport] Make the CharSequence wrappers in Predef non-implicit, for…

… JDK 15

In JDK 15 CharSequence has an isEmpty method with a default
implementation, which clashes with our Array[Char]#isEmpty,
IndexedSeq[Char]#isEmpty, as well as our StringBuilder and reflect's
Name.

Backport of scala#9292 from 2.13.x to 2.12.x

src/library/scala/Predef.scala

@@ -97,9 +97,9 @@ import scala.io.StdIn
  * @groupprio implicit-classes-any 70
  * @groupdesc implicit-classes-any These implicit classes add useful extension methods to every type.
  *
- * @groupname implicit-classes-char CharSequence Conversions
- * @groupprio implicit-classes-char 80
- * @groupdesc implicit-classes-char These implicit classes add CharSequence methods to Array[Char] and IndexedSeq[Char] instances.
+ * @groupname char-sequence-wrappers CharSequence Wrappers
+ * @groupprio char-sequence-wrappers 80
+ * @groupdesc char-sequence-wrappers Wrappers that implements CharSequence and were implicit classes.
  *
  * @groupname conversions-java-to-anyval Java to Scala
  * @groupprio conversions-java-to-anyval 90
@@ -349,22 +349,28 @@ object Predef extends LowPriorityImplicits with DeprecatedPredef {
   // and `@deprecatedName(Symbol("<none>"), "2.12.0")` crashes scalac with
   //   scala.reflect.internal.Symbols$CyclicReference: illegal cyclic reference involving object Symbol
   // in run/repl-no-imports-no-predef-power.scala.
-  /** @group implicit-classes-char */
-  implicit final class SeqCharSequence(@deprecated("will be made private", "2.12.0") @deprecatedName(null, "2.12.0") val __sequenceOfChars: scala.collection.IndexedSeq[Char]) extends CharSequence {
+  /** @group char-sequence-wrappers */
+  final class SeqCharSequence(@deprecated("will be made private", "2.12.0") @deprecatedName(null, "2.12.0") val __sequenceOfChars: scala.collection.IndexedSeq[Char]) extends CharSequence {
     def length: Int                                     = __sequenceOfChars.length
     def charAt(index: Int): Char                        = __sequenceOfChars(index)
     def subSequence(start: Int, end: Int): CharSequence = new SeqCharSequence(__sequenceOfChars.slice(start, end))
     override def toString                               = __sequenceOfChars mkString ""
   }
 
-  /** @group implicit-classes-char */
-  implicit final class ArrayCharSequence(@deprecated("will be made private", "2.12.0") @deprecatedName(null, "2.12.0") val __arrayOfChars: Array[Char]) extends CharSequence {
+  /** @group char-sequence-wrappers */
+  def SeqCharSequence(sequenceOfChars: scala.collection.IndexedSeq[Char]): SeqCharSequence = new SeqCharSequence(sequenceOfChars)
+
+  /** @group char-sequence-wrappers */
+  final class ArrayCharSequence(@deprecated("will be made private", "2.12.0") @deprecatedName(null, "2.12.0") val __arrayOfChars: Array[Char]) extends CharSequence {
     def length: Int                                     = __arrayOfChars.length
     def charAt(index: Int): Char                        = __arrayOfChars(index)
     def subSequence(start: Int, end: Int): CharSequence = new runtime.ArrayCharSequence(__arrayOfChars, start, end)
     override def toString                               = __arrayOfChars mkString ""
   }
 
+  /** @group char-sequence-wrappers */
+  def ArrayCharSequence(arrayOfChars: Array[Char]): ArrayCharSequence = new ArrayCharSequence(arrayOfChars)
+
   implicit val StringCanBuildFrom: CanBuildFrom[String, Char, String] = new CanBuildFrom[String, Char, String] {
     def apply(from: String) = apply()
     def apply()             = mutable.StringBuilder.newBuilder

src/library/scala/collection/mutable/StringBuilder.scala

@@ -447,6 +447,15 @@ final class StringBuilder(private val underlying: JavaStringBuilder)
    *  @return  the string assembled by this StringBuilder
    */
   def result(): String = toString
+
+
+  /** Tests whether this builder is empty.
+   *
+   *  This method is required for JDK15+ compatibility
+   *
+   *  @return  `true` if this builder contains nothing, `false` otherwise.
+   */
+  override def isEmpty: Boolean = underlying.length() == 0
 }
 
 object StringBuilder {

src/reflect/scala/reflect/internal/Names.scala

@@ -188,12 +188,17 @@ trait Names extends api.Names {
 
 // Classes ----------------------------------------------------------------------
 
+  // Dummy trait to make Name#isEmpty with override keyword at JDK before 15
+  sealed trait NameHasIsEmpty {
+    def isEmpty: Boolean
+  }
+
   /** The name class.
    *  TODO - resolve schizophrenia regarding whether to treat Names as Strings
    *  or Strings as Names.  Give names the key functions the absence of which
    *  make people want Strings all the time.
    */
-  sealed abstract class Name(protected val index: Int, protected val len: Int, protected val cachedString: String) extends NameApi with CharSequence {
+  sealed abstract class Name(protected val index: Int, protected val len: Int, protected val cachedString: String) extends NameApi with NameHasIsEmpty with CharSequence {
     type ThisNameType >: Null <: Name
     protected[this] def thisName: ThisNameType
 
@@ -209,8 +214,9 @@ trait Names extends api.Names {
 
     /** The length of this name. */
     final def length: Int = len
-    final def isEmpty = length == 0
     final def nonEmpty = !isEmpty
+    // This method is implements NameHasIsEmpty, and overrides CharSequence's isEmpty on JDK 15+
+    override final def isEmpty = length == 0
 
     def nameKind: String
     def isTermName: Boolean

src/reflect/scala/reflect/internal/StdNames.scala

@@ -77,7 +77,7 @@ trait StdNames {
       val suffix = s takeRight edge
 
       val cs = s.toArray
-      val bytes = Codec toUTF8 cs
+      val bytes = Codec.toUTF8(new scala.runtime.ArrayCharSequence(cs, 0, cs.length))
       md5 update bytes
       val md5chars = (md5.digest() map (b => (b & 0xFF).toHexString)).mkString
 

test/files/run/array-charSeq.scala

@@ -1,6 +1,6 @@
 object Test {
   val arr = Array[Char]('a' to 'i': _*)
-  var xs: CharSequence = arr
+  var xs: CharSequence = ArrayCharSequence(arr)
   val hash = xs.hashCode
 
   def check(chars: CharSequence) {

test/junit/scala/ArrayTest.scala

@@ -0,0 +1,30 @@
+package scala
+
+import org.junit.Assert.{ assertArrayEquals, assertFalse, assertTrue }
+import org.junit.Test
+
+import scala.runtime.BoxedUnit
+
+class ArrayTest {
+
+  @Test
+  def testArrayIsEmpty(): Unit = {
+    assertTrue(Array[Int]().isEmpty)
+    assertTrue(Array[Char]().isEmpty) // scala/bug#12172
+    assertTrue(Array[String]().isEmpty)
+
+    assertFalse(Array(1).isEmpty)
+    assertFalse(Array[Char](1).isEmpty)
+    assertFalse(Array("").isEmpty)
+
+    def ge[T](a: Array[T]) = a.isEmpty
+
+    assertTrue(ge(Array[Int]()))
+    assertTrue(ge(Array[Char]()))
+    assertTrue(ge(Array[String]()))
+
+    assertFalse(ge(Array(1)))
+    assertFalse(ge(Array[Char]('x')))
+    assertFalse(ge(Array("")))
+  }
+}