Merge pull request #9076 from lrytz/t12048

src/library/scala/collection/immutable/IntMap.scala

@@ -261,7 +261,7 @@ sealed abstract class IntMap[+T] extends AbstractMap[Int, T]
 
   override protected[this] def className = "IntMap"
 
-  override def isEmpty = this == IntMap.Nil
+  override def isEmpty = this eq IntMap.Nil
   override def knownSize: Int = if (isEmpty) 0 else super.knownSize
   override def filter(f: ((Int, T)) => Boolean): IntMap[T] = this match {
     case IntMap.Bin(prefix, mask, left, right) => {

src/library/scala/collection/immutable/LongMap.scala

@@ -256,7 +256,7 @@ sealed abstract class LongMap[+T] extends AbstractMap[Long, T]
 
   override protected[this] def className = "LongMap"
 
-  override def isEmpty = this == LongMap.Nil
+  override def isEmpty = this eq LongMap.Nil
   override def knownSize: Int = if (isEmpty) 0 else super.knownSize
   override def filter(f: ((Long, T)) => Boolean): LongMap[T] = this match {
     case LongMap.Bin(prefix, mask, left, right) => {

src/testkit/scala/tools/testkit/AssertUtil.scala

@@ -253,6 +253,18 @@ object AssertUtil {
   def readyOrNot(awaitable: Awaitable[_]): Boolean = Try(Await.ready(awaitable, TestDuration.Standard)).isSuccess
 
   def withoutATrace[A](body: => A) = NoTrace(body)
+
+  private lazy val modsField = classOf[Field].getDeclaredField("modifiers").tap(_.setAccessible(true))
+
+  def getFieldAccessible[T: ClassTag](n: String): Field =
+    implicitly[ClassTag[T]]
+      .runtimeClass.getDeclaredField(n)
+      .tap { f =>
+        if ((f.getModifiers & Modifier.FINAL) != 0)
+          modsField.setInt(f, f.getModifiers() & ~Modifier.FINAL)
+        if ((f.getModifiers & Modifier.PUBLIC) == 0)
+          f.setAccessible(true)
+      }
 }
 
 object TestDuration {

test/junit/scala/collection/immutable/IntMapTest.scala

@@ -0,0 +1,31 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.collection.immutable
+
+import org.junit.Assert._
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+import scala.tools.testkit.AssertUtil.{getFieldAccessible => f}
+
+@RunWith(classOf[JUnit4])
+class IntMapTest {
+  @Test
+  def `isEmpty O(1)`(): Unit = {
+    val m = IntMap(1 -> (), 2 -> (), 3 -> ())
+    f[IntMap.Bin[_]]("left").set(m, null)
+    f[IntMap.Bin[_]]("right").set(m, null)
+    assertFalse(m.isEmpty) // no NPE, does not access left or right
+  }
+}

test/junit/scala/collection/immutable/LongMapTest.scala

@@ -0,0 +1,32 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.collection.immutable
+
+import org.junit.Assert._
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+import scala.tools.testkit.AssertUtil.{getFieldAccessible => f}
+
+@RunWith(classOf[JUnit4])
+class LongMapTest {
+
+  @Test
+  def `isEmpty O(1)`(): Unit = {
+    val m = LongMap(1l -> (), 2l -> (), 3l -> ())
+    f[LongMap.Bin[_]]("left").set(m, null)
+    f[LongMap.Bin[_]]("right").set(m, null)
+    assertFalse(m.isEmpty) // no NPE, does not access left or right
+  }
+}