Don't evaluate suffix twice in LazyList#lazyAppendedAll

Port of scala/scala#8851. Co-authored-by: NthPortal <nthportal@gmail.com>
ijuma · Jul 5, 2020 · b595f47 · b595f47
1 parent 223bdd9
commit b595f47
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Showing 2 changed files with 10 additions and 2 deletions.
diff --git a/compat/src/main/scala-2.11_2.12/scala/collection/compat/immutable/LazyList.scala b/compat/src/main/scala-2.11_2.12/scala/collection/compat/immutable/LazyList.scala
@@ -365,7 +365,7 @@ final class LazyList[+A] private (private[this] var lazyState: () => LazyList.St
     newLL {
       if (isEmpty) suffix match {
         case lazyList: LazyList[B] => lazyList.state // don't recompute the LazyList
-        case _                     => stateFromIterator(suffix.toIterator)
+        case coll                  => stateFromIterator(coll.toIterator)
       } else sCons(head, tail lazyAppendedAll suffix)
     }
 

diff --git a/compat/src/test/scala/test/scala/collection/LazyListTest.scala b/compat/src/test/scala/test/scala/collection/LazyListTest.scala
@@ -355,7 +355,7 @@ class LazyListTest {
     assertEquals(1 to 10, build(_ ++= LazyList.from(1).take(10)))
     assertEquals(1 to 10, build(_ ++= Iterator.from(1).take(10)))
   }
-  
+
   @Test
   def selfReferentialFailure(): Unit = {
     def assertNoStackOverflow[A](lazyList: LazyList[A]): Unit = {
@@ -372,4 +372,12 @@ class LazyListTest {
     assertNoStackOverflow((new L).a)
     assertNoStackOverflow((new L).b)
   }
+
+  // scala/bug#11931
+  @Test
+  def lazyAppendedAllExecutesOnce(): Unit = {
+    var count = 0
+    LazyList(1).lazyAppendedAll({ count += 1; Seq(2)}).toList
+    assertEquals(1, count)
+  }
 }