[bug#3088][bug#12121] Fix adding/subtracting ListBuffer to/from itself

Fix appending a `ListBuffer` to itself and subtracting
a `ListBuffer` from itself.

Fix appending a `Growable` that uses the default
`addAll` implementation to itself.

src/library/scala/collection/mutable/Growable.scala

@@ -14,7 +14,7 @@ package scala
 package collection
 package mutable
 
-import scala.collection.IterableOnce
+import scala.annotation.nowarn
 
 /** This trait forms part of collections that can be augmented
   * using a `+=` operator and that can be cleared of all elements using
@@ -56,10 +56,14 @@ trait Growable[-A] extends Clearable {
    *  @param xs   the IterableOnce producing the elements to $add.
    *  @return  the $coll itself.
    */
+  @nowarn("msg=will most likely never compare equal")
   def addAll(xs: IterableOnce[A]): this.type = {
-    val it = xs.iterator
-    while (it.hasNext) {
-      addOne(it.next())
+    if (xs.asInstanceOf[AnyRef] eq this) addAll(Buffer.from(xs)) // avoid mutating under our own iterator
+    else {
+      val it = xs.iterator
+      while (it.hasNext) {
+        addOne(it.next())
+      }
     }
     this
   }

src/library/scala/collection/mutable/ListBuffer.scala

@@ -114,18 +114,28 @@ class ListBuffer[A]
 
   // Overridden for performance
   override final def addAll(xs: IterableOnce[A]): this.type = {
-    val it = xs.iterator
-    if (it.hasNext) {
-      ensureUnaliased()
-      val last1 = new ::[A](it.next(), Nil)
-      if (len == 0) first = last1 else last0.next = last1
-      last0 = last1
-      len += 1
-      while (it.hasNext) {
+    if (xs.asInstanceOf[AnyRef] eq this) { // avoid mutating under our own iterator
+      if (len > 0) {
+        ensureUnaliased()
+        val copy = ListBuffer.from(this)
+        last0.next = copy.first
+        last0 = copy.last0
+        len *= 2
+      }
+    } else {
+      val it = xs.iterator
+      if (it.hasNext) {
+        ensureUnaliased()
         val last1 = new ::[A](it.next(), Nil)
-        last0.next = last1
+        if (len == 0) first = last1 else last0.next = last1
         last0 = last1
         len += 1
+        while (it.hasNext) {
+          val last1 = new ::[A](it.next(), Nil)
+          last0.next = last1
+          last0 = last1
+          len += 1
+        }
       }
     }
     this
@@ -230,13 +240,29 @@ class ListBuffer[A]
   }
 
   def insertAll(idx: Int, elems: IterableOnce[A]): Unit = {
-    ensureUnaliased()
-    val it = elems.iterator
-    if (it.hasNext) {
-      ensureUnaliased()
-      if (idx < 0 || idx > len) throw new IndexOutOfBoundsException(s"$idx is out of bounds (min 0, max ${len-1})")
-      if (idx == len) ++=(elems)
-      else insertAfter(locate(idx), it)
+    if (idx < 0 || idx > len) throw new IndexOutOfBoundsException(s"$idx is out of bounds (min 0, max ${len-1})")
+    elems match {
+      case elems: AnyRef if elems eq this => // avoid mutating under our own iterator
+        if (len > 0) {
+          val copy = ListBuffer.from(this)
+          if (idx == 0 || idx == len) { // prepend/append
+            last0.next = copy.first
+            last0 = copy.last0
+          } else {
+            val prev = locate(idx) // cannot be `null` because other condition catches that
+            val follow = prev.next
+            prev.next = copy.first
+            copy.last0.next = follow
+          }
+          len *= 2
+        }
+      case elems =>
+        val it = elems.iterator
+        if (it.hasNext) {
+          ensureUnaliased()
+          if (idx == len) ++=(elems)
+          else insertAfter(locate(idx), it)
+        }
     }
   }
 

src/library/scala/collection/mutable/Shrinkable.scala

@@ -13,7 +13,7 @@
 package scala
 package collection.mutable
 
-import scala.annotation.tailrec
+import scala.annotation.{nowarn, tailrec}
 
 /** This trait forms part of collections that can be reduced
   *  using a `-=` operator.
@@ -52,6 +52,7 @@ trait Shrinkable[-A] {
     *  @param xs   the iterator producing the elements to remove.
     *  @return the $coll itself
     */
+  @nowarn("msg=will most likely never compare equal")
   def subtractAll(xs: collection.IterableOnce[A]): this.type = {
     @tailrec def loop(xs: collection.LinearSeq[A]): Unit = {
       if (xs.nonEmpty) {
@@ -59,9 +60,16 @@ trait Shrinkable[-A] {
         loop(xs.tail)
       }
     }
-    xs match {
-      case xs: collection.LinearSeq[A] => loop(xs)
-      case xs => xs.iterator.foreach(subtractOne)
+    if (xs.asInstanceOf[AnyRef] eq this) { // avoid mutating under our own iterator
+      xs match {
+        case xs: Clearable => xs.clear()
+        case xs            => subtractAll(Buffer.from(xs))
+      }
+    } else {
+      xs match {
+        case xs: collection.LinearSeq[A] => loop(xs)
+        case xs => xs.iterator.foreach(subtractOne)
+      }
     }
     this
   }

test/files/run/t3088.scala

@@ -2,8 +2,7 @@ import collection.mutable._
 
 object Test {
   def main(args: Array[String]): Unit = {
-    val b = new ListBuffer[Int]
-    b += 1
+    val b = ListBuffer(1, 2, 3)
     b ++= b
   }
 }

test/junit/scala/collection/mutable/ListBufferTest.scala

@@ -3,6 +3,8 @@ package scala.collection.mutable
 import org.junit.Assert.{assertEquals, assertTrue}
 import org.junit.Test
 
+import scala.tools.testkit.AssertUtil.assertSameElements
+
 import scala.annotation.nowarn
 
 class ListBufferTest {
@@ -228,4 +230,38 @@ class ListBufferTest {
     b += "b"
     assertEquals(Seq("a1", "b"), b)
   }
+
+  /* tests for scala/bug#12121 */
+
+  @Test
+  def self_addAll(): Unit = {
+    val b = ListBuffer(1, 2, 3)
+    b ++= b
+    assertSameElements(List(1, 2, 3, 1, 2, 3), b)
+  }
+
+  @Test
+  def self_prependAll(): Unit = {
+    val b = ListBuffer(1, 2, 3)
+    b prependAll b
+    assertSameElements(List(1, 2, 3, 1, 2, 3), b)
+  }
+
+  @Test
+  def self_insertAll(): Unit = {
+    val b1 = ListBuffer(1, 2, 3)
+    b1.insertAll(1, b1)
+    assertSameElements(List(1, 1, 2, 3, 2, 3), b1)
+
+    val b2 = ListBuffer(1, 2, 3)
+    b2.insertAll(3, b2)
+    assertSameElements(List(1, 2, 3, 1, 2, 3), b2)
+  }
+
+  @Test
+  def self_subtractAll(): Unit = {
+    val b = ListBuffer(1, 2, 3)
+    b --= b
+    assertSameElements(Nil, b)
+  }
 }