Fix unreported bug in View.Patched

Fix unreported bug in `View.Patched` where iterator
is incorrect due to the patch only being iterable once,
and already having been exhausted.

Simplify `ArrayBuffer#insertAll` code.

src/library/scala/collection/View.scala

@@ -404,9 +404,15 @@ object View extends IterableFactory[View] {
 
   @SerialVersionUID(3L)
   private[collection] class Patched[A](underlying: SomeIterableOps[A], from: Int, other: IterableOnce[A], replaced: Int) extends AbstractView[A] {
-    def iterator: Iterator[A] = underlying.iterator.patch(from, other.iterator, replaced)
+    // we may be unable to traverse `other` more than once, so we need to cache it if that's the case
+    private val _other: Iterable[A] = other match {
+      case other: Iterable[A] => other
+      case other              => LazyList.from(other)
+    }
+
+    def iterator: Iterator[A] = underlying.iterator.patch(from, _other.iterator, replaced)
     override def knownSize: Int = if (underlying.knownSize == 0 && other.knownSize == 0) 0 else super.knownSize
-    override def isEmpty: Boolean = if (knownSize == 0) true else iterator.isEmpty
+    override def isEmpty: Boolean = if (knownSize == 0) true else _other.isEmpty
   }
 
   @SerialVersionUID(3L)

src/library/scala/collection/mutable/ArrayBuffer.scala

@@ -177,27 +177,21 @@ class ArrayBuffer[A] private (initialElements: Array[AnyRef], initialSize: Int)
       case elems: collection.Iterable[A] =>
         val elemsLength = elems.size
         if (elemsLength > 0) {
-          ensureSize(length + elemsLength)
-          Array.copy(array, index, array, index + elemsLength, size0 - index)
-          size0 = size0 + elemsLength
-          elems match {
-            case elems: ArrayBuffer[_] =>
-              // if `elems eq this`, this works because `elems.array eq this.array`,
-              // we didn't overwrite the values being inserted after moving them in
-              // the previous copy a few lines up, and `System.arraycopy` will
-              // effectively "read" all the values before overwriting any of them.
-              Array.copy(elems.array, 0, array, index, elemsLength)
-            case _ =>
-              var i = 0
-              val it = elems.iterator
-              while (i < elemsLength) {
-                this(index + i) = it.next()
-                i += 1
-              }
-          }
+          val len = size0
+          val newSize = len + elemsLength
+          ensureSize(newSize)
+          Array.copy(array, index, array, index + elemsLength, len - index)
+          // if `elems eq this`, this copy is safe because
+          //   - `elems.array eq this.array`
+          //   - we didn't overwrite the values being inserted after moving them in
+          //     the previous line
+          //   - `copyElemsToArray` will call `System.arraycopy`
+          //   - `System.arraycopy` will effectively "read" all the values before
+          //     overwriting any of them when two arrays are the the same reference
+          IterableOnce.copyElemsToArray(elems, array.asInstanceOf[Array[Any]], index, elemsLength)
+          size0 = newSize // update size AFTER the copy, in case we're inserting a proxy
         }
-      case _ =>
-        insertAll(index, ArrayBuffer.from(elems))
+      case _ => insertAll(index, ArrayBuffer.from(elems))
     }
   }
 

test/junit/scala/collection/ViewTest.scala

@@ -113,4 +113,11 @@ class ViewTest {
   def _toString(): Unit = {
     assertEquals("View(<not computed>)", View(1, 2, 3).toString)
   }
+
+  // see https://github.com/scala/scala/pull/9388/files#r630681050
+  @Test
+  def patch(): Unit = {
+    val view = List(2).view.patch(1, List(3, 4, 5).iterator, 0)
+    assertEquals(view.toList, view.toList)
+  }
 }