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Merge pull request #10209 from retronym/build-from-divergence

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Jason Zaugg 2022-11-09 15:50:12 +10:00 committed by GitHub
parent 9854da2bd3 f2eee9cab0
commit bfedc2b966
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2 changed files with 43 additions and 3 deletions
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9
src/library/scala/collection/BuildFrom.scala
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@ -45,14 +45,14 @@ trait BuildFrom[-From, -A, +C] extends Any { self =>
object BuildFrom extends BuildFromLowPriority1 {
/** Build the source collection type from a MapOps */
implicit def buildFromMapOps[CC[X, Y] <: Map[X, Y] with MapOps[X, Y, CC, _], K0, V0, K, V]: BuildFrom[CC[K0, V0], (K, V), CC[K, V]] = new BuildFrom[CC[K0, V0], (K, V), CC[K, V]] {
implicit def buildFromMapOps[CC[X, Y] <: Map[X, Y] with MapOps[X, Y, CC, _], K0, V0, K, V]: BuildFrom[CC[K0, V0] with Map[K0, V0], (K, V), CC[K, V] with Map[K, V]] = new BuildFrom[CC[K0, V0], (K, V), CC[K, V]] {
//TODO: Reuse a prototype instance
def newBuilder(from: CC[K0, V0]): Builder[(K, V), CC[K, V]] = (from: MapOps[K0, V0, CC, _]).mapFactory.newBuilder[K, V]
def fromSpecific(from: CC[K0, V0])(it: IterableOnce[(K, V)]): CC[K, V] = (from: MapOps[K0, V0, CC, _]).mapFactory.from(it)
}
/** Build the source collection type from a SortedMapOps */
implicit def buildFromSortedMapOps[CC[X, Y] <: SortedMap[X, Y] with SortedMapOps[X, Y, CC, _], K0, V0, K : Ordering, V]: BuildFrom[CC[K0, V0], (K, V), CC[K, V]] = new BuildFrom[CC[K0, V0], (K, V), CC[K, V]] {
implicit def buildFromSortedMapOps[CC[X, Y] <: SortedMap[X, Y] with SortedMapOps[X, Y, CC, _], K0, V0, K : Ordering, V]: BuildFrom[CC[K0, V0] with SortedMap[K0, V0], (K, V), CC[K, V] with SortedMap[K, V]] = new BuildFrom[CC[K0, V0], (K, V), CC[K, V]] {
def newBuilder(from: CC[K0, V0]): Builder[(K, V), CC[K, V]] = (from: SortedMapOps[K0, V0, CC, _]).sortedMapFactory.newBuilder[K, V]
def fromSpecific(from: CC[K0, V0])(it: IterableOnce[(K, V)]): CC[K, V] = (from: SortedMapOps[K0, V0, CC, _]).sortedMapFactory.from(it)
}
@ -92,7 +92,10 @@ object BuildFrom extends BuildFromLowPriority1 {
trait BuildFromLowPriority1 extends BuildFromLowPriority2 {
/** Build the source collection type from an Iterable with SortedOps */
implicit def buildFromSortedSetOps[CC[X] <: SortedSet[X] with SortedSetOps[X, CC, _], A0, A : Ordering]: BuildFrom[CC[A0], A, CC[A]] = new BuildFrom[CC[A0], A, CC[A]] {
// Restating the upper bound of CC in the result type seems redundant, but it serves to prune the
// implicit search space for faster compilation and reduced change of divergence. See the compilation
// test in test/junit/scala/collection/BuildFromTest.scala and discussion in https://github.com/scala/scala/pull/10209
implicit def buildFromSortedSetOps[CC[X] <: SortedSet[X] with SortedSetOps[X, CC, _], A0, A : Ordering]: BuildFrom[CC[A0] with SortedSet[A0], A, CC[A] with SortedSet[A]] = new BuildFrom[CC[A0], A, CC[A]] {
def newBuilder(from: CC[A0]): Builder[A, CC[A]] = (from: SortedSetOps[A0, CC, _]).sortedIterableFactory.newBuilder[A]
def fromSpecific(from: CC[A0])(it: IterableOnce[A]): CC[A] = (from: SortedSetOps[A0, CC, _]).sortedIterableFactory.from(it)
}

37
test/junit/scala/collection/BuildFromTest.scala
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@ -176,4 +176,41 @@ class BuildFromTest {
immutable.LongMap: BuildFrom[_, (Long, String), immutable.LongMap[String]]
mutable.LongMap: BuildFrom[_, (Long, String), mutable.LongMap[String]]
mutable.AnyRefMap: BuildFrom[_, (String, String), mutable.AnyRefMap[String, String]]
// Check that we don't get an implicit divergence in a futile part of the search tree:
{
sealed trait GPoint
sealed trait HNil extends GPoint
class HCons[H, +T <: GPoint] extends GPoint
abstract class ExtendsOrdered extends Ordered[ExtendsOrdered]
// In scala 2.13, this implicit search considers BuildFrom.buildFromSortedSetOps
// which looks for a dep. implicit of type Ordering[(Int, HCons[ExtendsOrdered, HNil])]
implicitly[collection.BuildFrom[Seq[Any], (Int, HCons[ExtendsOrdered, HNil]), Seq[(Int, HCons[ExtendsOrdered, HNil])]]]
//
// In Scala 2.12, buildFromSortedSetOps is not a candidate because if it is in the companion object of
// the SortedSet heirarchy, which is not part of the implicit scope for this search.
// In 2.13, the implicit was moved to `object BuildFrom`, so _is_ considered
//
// The dependent implicit search:
//
// implicitly[(Int, HCons[ExtendsOrdered, HNil])]
//
// ... diverges on both Scala 2.12 and 2.13
//
// error: diverging implicit expansion for type scala.math.Ordering.AsComparable[(Int, HCons[ExtendsOrdered,HNil])]
// starting with method orderingToOrdered in object Ordered
//
// Divergences in Ordering implicits are a long standing problem, but I always thought it too hard to
// fix while retaining source compatibility.
//
// Removing `extends Ordered[X]` avoids divergence, but I'm not sure why. I diffed the -Vtyper log but
// can't figure out why that is relevant.
//
// (In the original code, ExtendsOrdered was actually scala.Enumeration.Value, which does extends Ordered.
//
//
}
}