SI-8372: unspliceable type printed in error message

The b8a76f688c6ce2a4c305da064303bb46b53be875 introduced ArrayOps.{unzip,
unzip3} methods. Both of those methods have ClassTags as context bounds on
their type parameters so they can create (and return) instances of Arrays.
The type inference for those methods is supposed to be guided by
implicit evidence that T <: (T1, T2) (or T <: (T1, T2, T3) in unzip3 case).
However, context bounds are desugared into implicit parameters that
prepended in front of implicit parameters declared in source code. That
means the implicit evidence won't have a chance to guide type inference
because it comes as last implicit parameter.

This commit desugars context bounds and puts them at the end of implicit
parameter list. This way type inference is guided properly and we get
expected compiler errors for missing class tags.

The change to parameters order breaks binary compatibility with respect
to 2.11.0-RC1. I added filters to our binary compatibility configuration
files. We can get rid of them as soon as 2.11.0 is out.

1 files changed, 17 insertions, 2 deletions

diff --git a/src/library/scala/collection/mutable/ArrayOps.scala b/src/library/scala/collection/mutable/ArrayOps.scala
index e342e134b4..00491ef20e 100644
--- a/src/library/scala/collection/mutable/ArrayOps.scala
+++ b/src/library/scala/collection/mutable/ArrayOps.scala
@@ -114,10 +114,16 @@ trait ArrayOps[T] extends Any with ArrayLike[T, Array[T]] with CustomParalleliza
    *  @tparam T2    the type of the second half of the element pairs
    *  @param asPair an implicit conversion which asserts that the element type
    *                of this Array is a pair.
+   *  @param ct1    a class tag for T1 type parameter that is required to create an instance
+   *                of Array[T1]
+   *  @param ct2    a class tag for T2 type parameter that is required to create an instance
+   *                of Array[T2]
    *  @return       a pair of Arrays, containing, respectively, the first and second half
    *                of each element pair of this Array.
    */
-  def unzip[T1: ClassTag, T2: ClassTag](implicit asPair: T => (T1, T2)): (Array[T1], Array[T2]) = {
+  // implementation NOTE: ct1 and ct2 can't be written as context bounds because desugared
+  // implicits are put in front of asPair parameter that is supposed to guide type inference
+  def unzip[T1, T2](implicit asPair: T => (T1, T2), ct1: ClassTag[T1], ct2: ClassTag[T2]): (Array[T1], Array[T2]) = {
     val a1 = new Array[T1](length)
     val a2 = new Array[T2](length)
     var i = 0
@@ -137,10 +143,19 @@ trait ArrayOps[T] extends Any with ArrayLike[T, Array[T]] with CustomParalleliza
    *  @tparam T3      the type of the third of three elements in the triple
    *  @param asTriple an implicit conversion which asserts that the element type
    *                  of this Array is a triple.
+   *  @param ct1    a class tag for T1 type parameter that is required to create an instance
+   *                of Array[T1]
+   *  @param ct2    a class tag for T2 type parameter that is required to create an instance
+   *                of Array[T2]
+   *  @param ct3    a class tag for T3 type parameter that is required to create an instance
+   *                of Array[T3]
    *  @return         a triple of Arrays, containing, respectively, the first, second, and third
    *                  elements from each element triple of this Array.
    */
-  def unzip3[T1: ClassTag, T2: ClassTag, T3: ClassTag](implicit asTriple: T => (T1, T2, T3)): (Array[T1], Array[T2], Array[T3]) = {
+  // implementation NOTE: ct1, ct2, ct3 can't be written as context bounds because desugared
+  // implicits are put in front of asPair parameter that is supposed to guide type inference
+  def unzip3[T1, T2, T3](implicit asTriple: T => (T1, T2, T3), ct1: ClassTag[T1], ct2: ClassTag[T2],
+    ct3: ClassTag[T3]): (Array[T1], Array[T2], Array[T3]) = {
     val a1 = new Array[T1](length)
     val a2 = new Array[T2](length)
     val a3 = new Array[T3](length)