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.

1 files changed, 125 insertions, 0 deletions

diff --git a/test/files/run/inferred-type-constructors.scala b/test/files/run/inferred-type-constructors.scala
new file mode 100644
index 0000000000..79a8653f68
--- /dev/null
+++ b/test/files/run/inferred-type-constructors.scala
@@ -0,0 +1,125 @@
+package p {
+  trait TCon[+CC[X]] {
+    def fPublic: CC[Int]                        = ???
+    private[p] def fPackagePrivate: CC[Int]     = ???
+    protected[p] def fPackageProtected: CC[Int] = ???
+  }
+  trait Iterable[+A] extends TCon[Iterable]
+  trait Set[A] extends Iterable[A] with TCon[Set]
+  trait Seq[+A] extends Iterable[A] with TCon[Seq]
+
+  private[p] abstract class AIterable[+A] extends Iterable[A]
+  private[p] abstract class ASeq[+A] extends AIterable[A] with Seq[A]
+  private[p] abstract class ASet[A] extends AIterable[A] with Set[A]
+
+  package m {
+    private[m] abstract class ASeq[A] extends p.ASeq[A] with Seq[A]
+    private[m] abstract class ASet[A] extends p.ASet[A] with Set[A]
+    trait Set[A] extends p.Set[A] with TCon[Set]
+    trait Seq[A] extends p.Seq[A] with TCon[Seq]
+    trait BitSet extends ASet[Int]
+    trait IntSeq extends ASeq[Int]
+  }
+
+  package i {
+    private[i] abstract class ASeq[+A] extends p.ASeq[A] with Seq[A]
+    private[i] abstract class ASet[A] extends p.ASet[A] with Set[A]
+    trait Set[A] extends p.Set[A] with TCon[Set]
+    trait Seq[+A] extends p.Seq[A] with TCon[Seq]
+    trait BitSet extends ASet[Int]
+    trait IntSeq extends ASeq[Int]
+  }
+}
+
+object Test {
+  import scala.reflect.runtime.universe._
+  // Complicated by the absence of usable type constructor type tags.
+  def extract[A, CC[X]](xs: CC[A]): CC[A] = xs
+  def whatis[T: TypeTag](x: T): Unit = {
+    val tpe = typeOf[T]
+    val access = tpe.typeSymbol.asInstanceOf[scala.reflect.internal.HasFlags].accessString.replaceAllLiterally("package ", "")
+    println(f"$access%15s $tpe")
+  }
+
+  trait IntIterable extends p.Iterable[Int]
+  trait IntSet extends p.Set[Int]
+  trait IntSeq extends p.Seq[Int]
+
+  trait MutableIntSet extends p.m.Set[Int]
+  trait MutableIntSeq extends p.m.Seq[Int]
+
+  trait ImmutableIntSet extends p.i.Set[Int]
+  trait ImmutableIntSeq extends p.i.Seq[Int]
+
+  def f1: IntIterable = null
+  def f2: IntSet = null
+  def f3: IntSeq = null
+
+  def g1: MutableIntSet = null
+  def g2: MutableIntSeq = null
+  def g3: p.m.BitSet = null
+
+  def h1: ImmutableIntSeq = null
+  def h2: p.i.BitSet = null
+  def h3: p.i.IntSeq = null
+
+  def main(args: Array[String]): Unit = {
+    whatis(extract(f1))
+    whatis(extract(f2))
+    whatis(extract(f3))
+    whatis(extract(g1))
+    whatis(extract(g2))
+    whatis(extract(g3))
+    whatis(extract(h1))
+    whatis(extract(h2))
+    whatis(extract(h3))
+
+    whatis(extract(if (true) f1 else f2))
+    whatis(extract(if (true) f1 else f3))
+    whatis(extract(if (true) f1 else g1))
+    whatis(extract(if (true) f1 else g2))
+    whatis(extract(if (true) f1 else g3))
+    whatis(extract(if (true) f1 else h1))
+    whatis(extract(if (true) f1 else h2))
+    whatis(extract(if (true) f1 else h3))
+    whatis(extract(if (true) f2 else f3))
+    whatis(extract(if (true) f2 else g1))
+    whatis(extract(if (true) f2 else g2))
+    whatis(extract(if (true) f2 else g3))
+    whatis(extract(if (true) f2 else h1))
+    whatis(extract(if (true) f2 else h2))
+    whatis(extract(if (true) f2 else h3))
+    whatis(extract(if (true) f3 else g1))
+    whatis(extract(if (true) f3 else g2))
+    whatis(extract(if (true) f3 else g3))
+    whatis(extract(if (true) f3 else h1))
+    whatis(extract(if (true) f3 else h2))
+    whatis(extract(if (true) f3 else h3))
+    whatis(extract(if (true) g1 else g2))
+    whatis(extract(if (true) g1 else g3))
+    whatis(extract(if (true) g1 else h1))
+    whatis(extract(if (true) g1 else h2))
+    whatis(extract(if (true) g1 else h3))
+    whatis(extract(if (true) g2 else g3))
+    whatis(extract(if (true) g2 else h1))
+    whatis(extract(if (true) g2 else h2))
+    whatis(extract(if (true) g2 else h3))
+    whatis(extract(if (true) g3 else h1))
+    whatis(extract(if (true) g3 else h2))
+    whatis(extract(if (true) g3 else h3))
+    whatis(extract(if (true) h1 else h2))
+    whatis(extract(if (true) h1 else h3))
+    whatis(extract(if (true) h2 else h3))
+
+    whatis(extract(Nil))
+    whatis(extract(Vector()))
+    whatis(extract(Map[Int,Int]()))
+    whatis(extract(Set[Int]()))
+    whatis(extract(Seq[Int]()))
+    whatis(extract(Array[Int]()))
+    whatis(extract(scala.collection.immutable.BitSet(1)))
+    whatis(extract("abc"))
+    whatis(extract(if (true) Stream(1) else List(1)))
+    whatis(extract(if (true) Seq(1) else Set(1)))
+  }
+}