SI-7335 Mandate that parents of Predef must be defined in Predef.scala

This avoids thorny issues of cyclic references and/or missing symbol errors
due to the way Predef is completed earlier than user classes, and due to the
addition of `import Predef._` to LowPriorityImplicits.scala. More details in
Lukas's comments in [2].

How did we bootstrap originally? Manually ordering source files,
either in the Ant Build [1], or later in Global [2]. But neither mechanism
remains in place (due, I suppose, to the difficulty in defending them
with a test case.) The reordering in Global was removed in [3].

After the change, we can compile the standard library without the
results of a prior compilation run on the classpath.

    rm -rf target/locker/library/classes/scala && export LIB=build/pack/lib; java -Xmx1G -cp $LIB/scala-library.jar:$LIB/scala-reflect.jar:$LIB/scala-compiler.jar:$LIB/forkjoin.jar scala.tools.nsc.Main @args.txt
    src/library/scala/SerialVersionUID.scala:15: warning: Implementation restriction: subclassing Classfile does not
    make your annotation visible at runtime.  If that is what
    you want, you must write the annotation class in Java.
    class SerialVersionUID(value: Long) extends scala.annotation.ClassfileAnnotation
          ^
    warning: there were 75 deprecation warning(s); re-run with -deprecation for details
    warning: there were 1 unchecked warning(s); re-run with -unchecked for details
    warning: there were 5 feature warning(s); re-run with -feature for details
    four warnings found

 Where @args.txt contains:

     -classpath
     lib/forkjoin.jar:target/locker/library/classes
     -sourcepath
     src/library
     -d
     target/locker/library/classes
     src/library/scala/Function3.scala
     src/library/scala/Console.scala
     <more of src/library/**, unsorted>

This change will break scala-virtualized, which will need to move
the contents of StandardEmbeddings.scala into Predef.scala.

[1] https://github.com/scala/scala/commit/c5441dc
[2] https://github.com/scala/scala/commit/e72f0c7
[3] https://github.com/scala/scala/pull/968

2 files changed, 86 insertions, 95 deletions

diff --git a/src/library/scala/LowPriorityImplicits.scala b/src/library/scala/LowPriorityImplicits.scala
deleted file mode 100644
index 535f1ac699..0000000000
--- a/src/library/scala/LowPriorityImplicits.scala
+++ /dev/null
@@ -1,95 +0,0 @@
-/*                     __                                               *\
-**     ________ ___   / /  ___     Scala API                            **
-**    / __/ __// _ | / /  / _ |    (c) 2002-2013, LAMP/EPFL             **
-**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
-** /____/\___/_/ |_/____/_/ | |                                         **
-**                          |/                                          **
-\*                                                                      */
-
-package scala
-
-import scala.collection.{ mutable, immutable, generic }
-import mutable.WrappedArray
-import immutable.WrappedString
-import generic.CanBuildFrom
-import scala.language.implicitConversions
-
-/** The `LowPriorityImplicits` class provides implicit values that
- *  are valid in all Scala compilation units without explicit qualification,
- *  but that are partially overridden by higher-priority conversions in object
- *  `Predef`.
- *
- *  @author  Martin Odersky
- *  @since 2.8
- */
-private[scala] abstract class LowPriorityImplicits {
-  /** We prefer the java.lang.* boxed types to these wrappers in
-   *  any potential conflicts.  Conflicts do exist because the wrappers
-   *  need to implement ScalaNumber in order to have a symmetric equals
-   *  method, but that implies implementing java.lang.Number as well.
-   *
-   *  Note - these are inlined because they are value classes, but
-   *  the call to xxxWrapper is not eliminated even though it does nothing.
-   *  Even inlined, every call site does a no-op retrieval of Predef's MODULE$
-   *  because maybe loading Predef has side effects!
-   */
-  @inline implicit def byteWrapper(x: Byte)       = new runtime.RichByte(x)
-  @inline implicit def shortWrapper(x: Short)     = new runtime.RichShort(x)
-  @inline implicit def intWrapper(x: Int)         = new runtime.RichInt(x)
-  @inline implicit def charWrapper(c: Char)       = new runtime.RichChar(c)
-  @inline implicit def longWrapper(x: Long)       = new runtime.RichLong(x)
-  @inline implicit def floatWrapper(x: Float)     = new runtime.RichFloat(x)
-  @inline implicit def doubleWrapper(x: Double)   = new runtime.RichDouble(x)
-  @inline implicit def booleanWrapper(x: Boolean) = new runtime.RichBoolean(x)
-
-  // These eight implicits exist solely to exclude Null from the domain of
-  // the boxed types, so that e.g. "var x: Int = null" is a compile time
-  // error rather than a delayed null pointer exception by way of the
-  // conversion from java.lang.Integer.  If defined in the same file as
-  // Integer2int, they would have higher priority because Null is a subtype
-  // of Integer.  We balance that out and create conflict by moving the
-  // definition into the superclass.
-  //
-  // Caution: do not adjust tightrope tension without safety goggles in place.
-  implicit def Byte2byteNullConflict(x: Null): Byte          = sys.error("value error")
-  implicit def Short2shortNullConflict(x: Null): Short       = sys.error("value error")
-  implicit def Character2charNullConflict(x: Null): Char     = sys.error("value error")
-  implicit def Integer2intNullConflict(x: Null): Int         = sys.error("value error")
-  implicit def Long2longNullConflict(x: Null): Long          = sys.error("value error")
-  implicit def Float2floatNullConflict(x: Null): Float       = sys.error("value error")
-  implicit def Double2doubleNullConflict(x: Null): Double    = sys.error("value error")
-  implicit def Boolean2booleanNullConflict(x: Null): Boolean = sys.error("value error")
-
-  implicit def genericWrapArray[T](xs: Array[T]): WrappedArray[T] =
-    if (xs eq null) null
-    else WrappedArray.make(xs)
-
-  // Since the JVM thinks arrays are covariant, one 0-length Array[AnyRef]
-  // is as good as another for all T <: AnyRef.  Instead of creating 100,000,000
-  // unique ones by way of this implicit, let's share one.
-  implicit def wrapRefArray[T <: AnyRef](xs: Array[T]): WrappedArray[T] = {
-    if (xs eq null) null
-    else if (xs.length == 0) WrappedArray.empty[T]
-    else new WrappedArray.ofRef[T](xs)
-  }
-
-  implicit def wrapIntArray(xs: Array[Int]): WrappedArray[Int] = if (xs ne null) new WrappedArray.ofInt(xs) else null
-  implicit def wrapDoubleArray(xs: Array[Double]): WrappedArray[Double] = if (xs ne null) new WrappedArray.ofDouble(xs) else null
-  implicit def wrapLongArray(xs: Array[Long]): WrappedArray[Long] = if (xs ne null) new WrappedArray.ofLong(xs) else null
-  implicit def wrapFloatArray(xs: Array[Float]): WrappedArray[Float] = if (xs ne null) new WrappedArray.ofFloat(xs) else null
-  implicit def wrapCharArray(xs: Array[Char]): WrappedArray[Char] = if (xs ne null) new WrappedArray.ofChar(xs) else null
-  implicit def wrapByteArray(xs: Array[Byte]): WrappedArray[Byte] = if (xs ne null) new WrappedArray.ofByte(xs) else null
-  implicit def wrapShortArray(xs: Array[Short]): WrappedArray[Short] = if (xs ne null) new WrappedArray.ofShort(xs) else null
-  implicit def wrapBooleanArray(xs: Array[Boolean]): WrappedArray[Boolean] = if (xs ne null) new WrappedArray.ofBoolean(xs) else null
-  implicit def wrapUnitArray(xs: Array[Unit]): WrappedArray[Unit] = if (xs ne null) new WrappedArray.ofUnit(xs) else null
-
-  implicit def wrapString(s: String): WrappedString = if (s ne null) new WrappedString(s) else null
-  implicit def unwrapString(ws: WrappedString): String = if (ws ne null) ws.self else null
-
-  implicit def fallbackStringCanBuildFrom[T]: CanBuildFrom[String, T, immutable.IndexedSeq[T]] =
-    new CanBuildFrom[String, T, immutable.IndexedSeq[T]] {
-      def apply(from: String) = immutable.IndexedSeq.newBuilder[T]
-      def apply() = immutable.IndexedSeq.newBuilder[T]
-    }
-}
-
diff --git a/src/library/scala/Predef.scala b/src/library/scala/Predef.scala
index 9a468489a2..511e5b842a 100644
--- a/src/library/scala/Predef.scala
+++ b/src/library/scala/Predef.scala
@@ -446,3 +446,89 @@ private[scala] trait DeprecatedPredef {
   @deprecated("Use the method in `scala.io.ReadStdin`", "2.11.0") def readf2(format: String)             = ReadStdin.readf2(format)
   @deprecated("Use the method in `scala.io.ReadStdin`", "2.11.0") def readf3(format: String)             = ReadStdin.readf3(format)
 }
+
+/** The `LowPriorityImplicits` class provides implicit values that
+*  are valid in all Scala compilation units without explicit qualification,
+*  but that are partially overridden by higher-priority conversions in object
+*  `Predef`.
+*
+*  @author  Martin Odersky
+*  @since 2.8
+*/
+// SI-7335 Parents of Predef are defined in the same compilation unit to avoid
+// cyclic reference errors compiling the standard library *without* a previously
+// compiled copy on the classpath.
+private[scala] abstract class LowPriorityImplicits {
+  import mutable.WrappedArray
+  import immutable.WrappedString
+
+  /** We prefer the java.lang.* boxed types to these wrappers in
+   *  any potential conflicts.  Conflicts do exist because the wrappers
+   *  need to implement ScalaNumber in order to have a symmetric equals
+   *  method, but that implies implementing java.lang.Number as well.
+   *
+   *  Note - these are inlined because they are value classes, but
+   *  the call to xxxWrapper is not eliminated even though it does nothing.
+   *  Even inlined, every call site does a no-op retrieval of Predef's MODULE$
+   *  because maybe loading Predef has side effects!
+   */
+  @inline implicit def byteWrapper(x: Byte)       = new runtime.RichByte(x)
+  @inline implicit def shortWrapper(x: Short)     = new runtime.RichShort(x)
+  @inline implicit def intWrapper(x: Int)         = new runtime.RichInt(x)
+  @inline implicit def charWrapper(c: Char)       = new runtime.RichChar(c)
+  @inline implicit def longWrapper(x: Long)       = new runtime.RichLong(x)
+  @inline implicit def floatWrapper(x: Float)     = new runtime.RichFloat(x)
+  @inline implicit def doubleWrapper(x: Double)   = new runtime.RichDouble(x)
+  @inline implicit def booleanWrapper(x: Boolean) = new runtime.RichBoolean(x)
+
+  // These eight implicits exist solely to exclude Null from the domain of
+  // the boxed types, so that e.g. "var x: Int = null" is a compile time
+  // error rather than a delayed null pointer exception by way of the
+  // conversion from java.lang.Integer.  If defined in the same file as
+  // Integer2int, they would have higher priority because Null is a subtype
+  // of Integer.  We balance that out and create conflict by moving the
+  // definition into the superclass.
+  //
+  // Caution: do not adjust tightrope tension without safety goggles in place.
+  implicit def Byte2byteNullConflict(x: Null): Byte          = sys.error("value error")
+  implicit def Short2shortNullConflict(x: Null): Short       = sys.error("value error")
+  implicit def Character2charNullConflict(x: Null): Char     = sys.error("value error")
+  implicit def Integer2intNullConflict(x: Null): Int         = sys.error("value error")
+  implicit def Long2longNullConflict(x: Null): Long          = sys.error("value error")
+  implicit def Float2floatNullConflict(x: Null): Float       = sys.error("value error")
+  implicit def Double2doubleNullConflict(x: Null): Double    = sys.error("value error")
+  implicit def Boolean2booleanNullConflict(x: Null): Boolean = sys.error("value error")
+
+  implicit def genericWrapArray[T](xs: Array[T]): WrappedArray[T] =
+    if (xs eq null) null
+    else WrappedArray.make(xs)
+
+  // Since the JVM thinks arrays are covariant, one 0-length Array[AnyRef]
+  // is as good as another for all T <: AnyRef.  Instead of creating 100,000,000
+  // unique ones by way of this implicit, let's share one.
+  implicit def wrapRefArray[T <: AnyRef](xs: Array[T]): WrappedArray[T] = {
+    if (xs eq null) null
+    else if (xs.length == 0) WrappedArray.empty[T]
+    else new WrappedArray.ofRef[T](xs)
+  }
+
+  implicit def wrapIntArray(xs: Array[Int]): WrappedArray[Int] = if (xs ne null) new WrappedArray.ofInt(xs) else null
+  implicit def wrapDoubleArray(xs: Array[Double]): WrappedArray[Double] = if (xs ne null) new WrappedArray.ofDouble(xs) else null
+  implicit def wrapLongArray(xs: Array[Long]): WrappedArray[Long] = if (xs ne null) new WrappedArray.ofLong(xs) else null
+  implicit def wrapFloatArray(xs: Array[Float]): WrappedArray[Float] = if (xs ne null) new WrappedArray.ofFloat(xs) else null
+  implicit def wrapCharArray(xs: Array[Char]): WrappedArray[Char] = if (xs ne null) new WrappedArray.ofChar(xs) else null
+  implicit def wrapByteArray(xs: Array[Byte]): WrappedArray[Byte] = if (xs ne null) new WrappedArray.ofByte(xs) else null
+  implicit def wrapShortArray(xs: Array[Short]): WrappedArray[Short] = if (xs ne null) new WrappedArray.ofShort(xs) else null
+  implicit def wrapBooleanArray(xs: Array[Boolean]): WrappedArray[Boolean] = if (xs ne null) new WrappedArray.ofBoolean(xs) else null
+  implicit def wrapUnitArray(xs: Array[Unit]): WrappedArray[Unit] = if (xs ne null) new WrappedArray.ofUnit(xs) else null
+
+  implicit def wrapString(s: String): WrappedString = if (s ne null) new WrappedString(s) else null
+  implicit def unwrapString(ws: WrappedString): String = if (ws ne null) ws.self else null
+
+  implicit def fallbackStringCanBuildFrom[T]: CanBuildFrom[String, T, immutable.IndexedSeq[T]] =
+    new CanBuildFrom[String, T, immutable.IndexedSeq[T]] {
+      def apply(from: String) = immutable.IndexedSeq.newBuilder[T]
+      def apply() = immutable.IndexedSeq.newBuilder[T]
+    }
+}
+