Don't evaluate suffix twice in LazyList#lazyAppendedAll

Port of scala/scala#8851.

Co-authored-by: NthPortal <[email protected]>

Author: ijuma

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)
     }
 

compat/src/test/scala/test/scala/collection/LazyListTest.scala

@@ -376,4 +376,12 @@ 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)
+  }
 }