Fix ArrayIndexOutOfBoundsException in mutable.BitSet.<<=

Author: linasm

src/main/scala/scala/collection/decorators/MutableBitSetDecorator.scala

@@ -39,14 +39,16 @@ class MutableBitSetDecorator(protected val bs: mutable.BitSet) {
 
   private def shiftLeftInPlace(shiftBy: Int): Unit = {
 
-    val bitOffset = shiftBy & WordMask
-    val wordOffset = shiftBy >>> LogWL
-
     var significantWordCount = bs.nwords
     while (significantWordCount > 0 && bs.word(significantWordCount - 1) == 0) {
       significantWordCount -= 1
     }
 
+    if (significantWordCount == 0) return
+
+    val bitOffset = shiftBy & WordMask
+    val wordOffset = shiftBy >>> LogWL
+
     if (bitOffset == 0) {
       val newSize = significantWordCount + wordOffset
       require(newSize <= MaxSize)

src/test/scala/scala/collection/decorators/MutableBitSetDecoratorTest.scala

@@ -2,11 +2,12 @@ package scala.collection.decorators
 
 import org.junit.{Assert, Test}
 
+import scala.collection.BitSetOps
 import scala.collection.mutable.BitSet
 
 class MutableBitSetDecoratorTest {
 
-  import Assert.{assertEquals, assertSame}
+  import Assert.{assertEquals, assertSame, assertTrue}
   import BitSet.empty
 
   @Test
@@ -104,4 +105,12 @@ class MutableBitSetDecoratorTest {
     }
   }
 
+  @Test
+  def shiftLeftTwoEmptyWords(): Unit = {
+    val twoWords = BitSet(BitSetOps.WordLength + 1)
+    twoWords ^= twoWords
+    twoWords <<= 1
+    assertTrue(twoWords.isEmpty)
+  }
+
 }