Commits on Feb 6, 2014

1. SI-6948 Make the Abstract* classes public. …

   Several weaknesses in the implementation converge and force multiply.
   
   1) Type constructor inference is not persistent. During implicit search
   it will give up after the first seen parent even if some deeper base type
   (even another direct parent) would satisfy the search.
   
   2) Type inference is not aware of access restrictions. Inferred types are
   calculated with disregard for whether the inferred type is visible at
   the point of inference. That means that package-private types - which may be
   private for any number of good reasons, such as not wanting them to appear in
   bytecode thus creating binary compatibility obligations - are not private.
   There is no such thing as a qualified private type.
   
     package p {
       trait PublicInterface[T] { def foo(): Int }
       private[p] trait ImplementationOnly[T] extends PublicInterface[T] { def foo(): Int = 1 }
       class PublicClass extends ImplementationOnly[PublicClass]
     }
     package q {
       object Test {
         def f[A, CC[X]](xs: CC[A]): CC[A] = xs
         def g = f(new p.PublicClass) // inferred type: p.ImplementationOnly[p.PublicClass]
         def h = g.foo()
         // Bytecode contains:
         // public p.ImplementationOnly<p.PublicClass> g();
         // public int h();
         //    0: aload_0
         //    1: invokevirtual #30                 // Method g:()Lp/ImplementationOnly;
         //    4: invokeinterface #33,  1           // InterfaceMethod p/ImplementationOnly.foo:()I
         //    9: ireturn
       }
     }
   
   3) The trait encoding leads to a proliferation of forwarder methods, so much so that
   1.5 Mb of bytecode was taken off of the standard library size by creating abstract classes
   which act as central mixin points so that leaf classes can inherit some methods the
   old fashioned way rather than each receiving their own copy of every trait defined method.
   This was done for 2.10 through the creation of the Abstract* classes, all of which were
   given reduced visibility to keep them out of the API.
   
     private[collection] class AbstractSeq extends ...
   
   This achieved its intended goal very nicely, but also some unintended ones.
   
   In combination with 1) above:
   
     scala> val rand = new scala.util.Random()
     rand: scala.util.Random = scala.util.Random@7f85a53b
   
     // this works
     scala> rand.shuffle(0 to 5)
     res1: scala.collection.immutable.IndexedSeq[Int] = Vector(4, 0, 1, 2, 5, 3)
   
     // and this doesn't! good luck reasoning that one out
     scala> rand.shuffle(0 until 5)
     <console>:9: error: Cannot construct a collection of type scala.collection.AbstractSeq[Int]
       with elements of type Int based on a collection of type scala.collection.AbstractSeq[Int].
                   rand.shuffle(0 until 5)
                               ^
   
     // Somewhat comically, in scala 2.9 it was flipped: to failed (differently), until worked.
     scala> scala.util.Random.shuffle(0 to 5)
     <console>:8: error: type mismatch;
      found   : scala.collection.immutable.Range.Inclusive
      required: ?CC[?T]
   
     scala> scala.util.Random.shuffle(0 until 5)
     res2: scala.collection.immutable.IndexedSeq[Int] = Vector(4, 3, 1, 2, 0)
   
   In combination with 2) above:
   
     scala> def f[A, CC[X]](xs: CC[A]): CC[A] = xs
     f: [A, CC[X]](xs: CC[A])CC[A]
   
     scala> var x = f(1 until 10)
     x: scala.collection.AbstractSeq[Int] = Range(1, 2, 3, 4, 5, 6, 7, 8, 9)
   
     // It has inferred a type for our value which it will not allow us to use or even to reference.
     scala> var y: scala.collection.AbstractSeq[Int] = x
     <console>:10: error: class AbstractSeq in package collection cannot be accessed in package collection
            var y: scala.collection.AbstractSeq[Int] = x
                                    ^
     // This one is a straight regression - in scala 2.9,
     scala> var x = f(1 until 10)
     x: scala.collection.immutable.IndexedSeq[Int] = Range(1, 2, 3, 4, 5, 6, 7, 8, 9)
   
   Since 1) and 2) are essentially unfixable - at least by me - I propose
   to ameliorate these regressions by attacking the symptoms at the leaves.
   That means making all the Abstract* classes public - keeping in mind that
   they must already be assumed to be in the binary compatibility footprint,
   since they have been leaking throughout their existence. This only impacts
   the inference of inaccessible collections types - it doesn't help with the
   more serious issue with type inference.

   

   paulp committed Feb 6, 2014
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