From 8798098e070ead9687a994babe0ebb2b99c3d169 Mon Sep 17 00:00:00 2001
From: Jamie Thompson <bishbashboshjt@gmail.com>
Date: Fri, 5 Dec 2025 12:26:10 +0100
Subject: [PATCH] Add an implementation of BuildFrom for IArray

for use with e.g. Future.sequence, LazyZip, Random.shuffle
---
 library/src/scala/collection/BuildFrom.scala  | 10 +++++++
 project/MiMaFilters.scala                     |  2 ++
 .../stdlibExperimentalDefinitions.scala       |  3 +++
 .../IArrayOps-experimental-BuildFrom.scala    | 26 +++++++++++++++++++
 4 files changed, 41 insertions(+)
 create mode 100644 tests/run/IArrayOps-experimental-BuildFrom.scala

diff --git a/library/src/scala/collection/BuildFrom.scala b/library/src/scala/collection/BuildFrom.scala
index cb819db200ec..a7dd4b8856fa 100644
--- a/library/src/scala/collection/BuildFrom.scala
+++ b/library/src/scala/collection/BuildFrom.scala
@@ -19,6 +19,7 @@ import scala.annotation.implicitNotFound
 import scala.collection.mutable.Builder
 import scala.collection.immutable.WrappedString
 import scala.reflect.ClassTag
+import scala.annotation.experimental
 
 /** Builds a collection of type `C` from elements of type `A` when a source collection of type `From` is available.
   * Implicit instances of `BuildFrom` are available for all collection types.
@@ -84,6 +85,15 @@ object BuildFrom extends BuildFromLowPriority1 {
       def newBuilder(from: Array[?]): Builder[A, Array[A]] = Factory.arrayFactory[A].newBuilder
     }
 
+  @experimental
+  given buildFromIArray[A : ClassTag]: BuildFrom[IArray[Any], A, IArray[A]] =
+    // IArray[?] is unreducible wildcard, but as IArray is covariant its ok to use IArray[Any] here.
+    new BuildFrom[IArray[Any], A, IArray[A]]:
+      def fromSpecific(from: IArray[Any])(it: IterableOnce[A]^): IArray[A] =
+        // TODO: when IArray factory is added to library, delegate there.
+        IArray.unsafeFromArray(Factory.arrayFactory[A].fromSpecific(it))
+      def newBuilder(from: IArray[Any]): Builder[A, IArray[A]] = IArray.newBuilder[A]
+
   implicit def buildFromView[A, B]: BuildFrom[View[A], B, View[B]] =
     new BuildFrom[View[A], B, View[B]] {
       def fromSpecific(from: View[A])(it: IterableOnce[B]^): View[B]^{it} = View.from(it)
diff --git a/project/MiMaFilters.scala b/project/MiMaFilters.scala
index 9a633397924a..14a6ec678aeb 100644
--- a/project/MiMaFilters.scala
+++ b/project/MiMaFilters.scala
@@ -9,6 +9,8 @@ object MiMaFilters {
       // Additions that require a new minor version of the library
       Build.mimaPreviousDottyVersion -> Seq(
         ProblemFilters.exclude[DirectMissingMethodProblem]("scala.caps.package#package.freeze"),
+        // IArray integration with Scala Collections:
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.BuildFrom.buildFromIArray"),
       ),
 
     )
diff --git a/tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala b/tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala
index 4cfb23e229ac..8c32537b5d76 100644
--- a/tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala
+++ b/tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala
@@ -98,6 +98,9 @@ val experimentalDefinitionInLibrary = Set(
 
   // New feature: Erased trait
   "scala.compiletime.Erased",
+
+  // New API: BuildFrom[IArray[Any], T, IArray[T]]
+  "scala.collection.BuildFrom$.buildFromIArray",
 )
 
 
diff --git a/tests/run/IArrayOps-experimental-BuildFrom.scala b/tests/run/IArrayOps-experimental-BuildFrom.scala
new file mode 100644
index 000000000000..87b1941a48c9
--- /dev/null
+++ b/tests/run/IArrayOps-experimental-BuildFrom.scala
@@ -0,0 +1,26 @@
+//> using options -experimental
+
+import scala.collection.BuildFrom
+
+def diffs(ns: IArray[Long]): IArray[Long] =
+  ns.lazyZip(ns.tail).map((a, b) => b - a)
+
+def diffsB(ns: IArray[Long])(bf: BuildFrom[IArray[Long], Long, IArray[Long]]): IArray[Long] =
+  ns.lazyZip(ns.tail).map((a, b) => b - a)(using bf)
+
+@main def Test: Unit =
+  val explicit: BuildFrom[IArray[Long], Long, IArray[Long]] =
+    scala.collection.BuildFrom.buildFromIArray[Long]
+
+  val contextual: BuildFrom[IArray[Long], Long, IArray[Long]] =
+    summon[BuildFrom[IArray[Long], Long, IArray[Long]]]
+
+  val contextualWide: BuildFrom[IArray[Any], Long, IArray[Long]] =
+    summon[BuildFrom[IArray[Any], Long, IArray[Long]]]
+
+  assert(explicit.getClass == contextual.getClass) // check default is same implementation.
+  assert(explicit.getClass == contextualWide.getClass) // check default is same implementation.
+
+  assert(diffs(IArray(1L, 3L, 6L, 10L)).sameElements(IArray(2L, 3L, 4L)))
+  assert(diffsB(IArray(1L, 3L, 6L, 10L))(contextual).sameElements(IArray(2L, 3L, 4L)))
+  assert(diffsB(IArray(1L, 3L, 6L, 10L))(contextualWide).sameElements(IArray(2L, 3L, 4L)))