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

1 files changed, 9 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 0be7192471..be59b4e86b 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -1856,6 +1856,9 @@ trait Typers extends Adaptations with Tags {
       }
       val impl2  = finishMethodSynthesis(impl1, clazz, context)
 
+      if (mdef.symbol == PredefModule)
+        ensurePredefParentsAreInSameSourceFile(impl2)
+
       // SI-5954. On second compile of a companion class contained in a package object we end up
       // with some confusion of names which leads to having two symbols with the same name in the
       // same owner. Until that can be straightened out we will warn on companion objects in package
@@ -1884,6 +1887,12 @@ trait Typers extends Adaptations with Tags {
 
       treeCopy.ModuleDef(mdef, typedMods, mdef.name, impl2) setType NoType
     }
+
+    private def ensurePredefParentsAreInSameSourceFile(template: Template) = {
+      val parentSyms = template.parents map (_.symbol) filterNot (_ == AnyRefClass)
+      if (parentSyms exists (_.associatedFile != PredefModule.associatedFile))
+        unit.error(template.pos, s"All parents of Predef must be defined in ${PredefModule.associatedFile}.")
+    }
     /** In order to override this in the TreeCheckers Typer so synthetics aren't re-added
      *  all the time, it is exposed here the module/class typing methods go through it.
      *  ...but it turns out it's also the ideal spot for namer/typer coordination for