patch 8.2.2738: extending a list with itself can give wrong result

Problem:    Extending a list with itself can give wrong result.
Solution:   Remember the item before where the insertion happens and skip to
            after the already inserted items. (closes #1112)

src/list.c

@@ -894,6 +894,7 @@ list_extend(list_T *l1, list_T *l2, listitem_T *bef)
 {
     listitem_T	*item;
     int		todo;
+    listitem_T	*bef_prev;
 
     // NULL list is equivalent to an empty list: nothing to do.
     if (l2 == NULL || l2->lv_len == 0)
@@ -903,9 +904,15 @@ list_extend(list_T *l1, list_T *l2, listitem_T *bef)
     CHECK_LIST_MATERIALIZE(l1);
     CHECK_LIST_MATERIALIZE(l2);
 
+    // When exending a list with itself, at some point we run into the item
+    // that was before "bef" and need to skip over the already inserted items
+    // to "bef".
+    bef_prev = bef == NULL ? NULL : bef->li_prev;
+
     // We also quit the loop when we have inserted the original item count of
     // the list, avoid a hang when we extend a list with itself.
-    for (item = l2->lv_first; item != NULL && --todo >= 0; item = item->li_next)
+    for (item = l2->lv_first; item != NULL && --todo >= 0;
+				 item = item == bef_prev ? bef : item->li_next)
 	if (list_insert_tv(l1, &item->li_tv, bef) == FAIL)
 	    return FAIL;
     return OK;

src/testdir/test_listdict.vim

@@ -862,6 +862,20 @@ func Test_listdict_extend()
 
   " Extend g: dictionary with an invalid variable name
   call assert_fails("call extend(g:, {'-!' : 10})", 'E461:')
+
+  " Extend a list with itself.
+  let l = [1, 5, 7]
+  call extend(l, l, 0)
+  call assert_equal([1, 5, 7, 1, 5, 7], l)
+  let l = [1, 5, 7]
+  call extend(l, l, 1)
+  call assert_equal([1, 1, 5, 7, 5, 7], l)
+  let l = [1, 5, 7]
+  call extend(l, l, 2)
+  call assert_equal([1, 5, 1, 5, 7, 7], l)
+  let l = [1, 5, 7]
+  call extend(l, l, 3)
+  call assert_equal([1, 5, 7, 1, 5, 7], l)
 endfunc
 
 func Test_listdict_extendnew()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2738,
 /**/
     2737,
 /**/