4 files changed, 49 insertions, 13 deletions
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 9a5ab51c71..a1caeea95a 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -128,6 +128,7 @@ return {
     finddir={args={1, 3}, base=1},
     findfile={args={1, 3}, base=1},
     flatten={args={1, 2}, base=1},
+    flattennew={args={1, 2}, base=1},
     float2nr={args=1, base=1},
     floor={args=1, base=1, float_func="floor"},
     fmod={args=2, base=1},
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 6f983d3208..3a58b90f96 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -1870,8 +1870,8 @@ static void f_expandcmd(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   rettv->vval.v_string = cmdstr;
 }
 
-/// "flatten(list[, {maxdepth}])" function
-static void f_flatten(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
+/// "flatten()" and "flattennew()" functions
+static void flatten_common(typval_T *argvars, typval_T *rettv, bool make_copy)
 {
   bool error = false;
 
@@ -1895,13 +1895,38 @@ static void f_flatten(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   }
 
   list_T *list = argvars[0].vval.v_list;
-  if (list != NULL
-      && !value_check_lock(tv_list_locked(list),
-                           N_("flatten() argument"),
-                           TV_TRANSLATE)
-      && tv_list_flatten(list, maxdepth) == OK) {
-    tv_copy(&argvars[0], rettv);
+  rettv->v_type = VAR_LIST;
+  rettv->vval.v_list = list;
+  if (list == NULL) {
+    return;
   }
+
+  if (make_copy) {
+    list = tv_list_copy(NULL, list, true, get_copyID());
+    rettv->vval.v_list = list;
+    if (list == NULL) {
+      return;
+    }
+  } else {
+    if (value_check_lock(tv_list_locked(list), N_("flatten() argument"), TV_TRANSLATE)) {
+      return;
+    }
+    tv_list_ref(list);
+  }
+
+  tv_list_flatten(list, maxdepth);
+}
+
+/// "flatten(list[, {maxdepth}])" function
+static void f_flatten(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
+{
+  flatten_common(argvars, rettv, false);
+}
+
+/// "flattennew(list[, {maxdepth}])" function
+static void f_flattennew(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
+{
+  flatten_common(argvars, rettv, true);
 }
 
 /// "extend(list, list [, idx])" function
diff --git a/src/nvim/eval/typval.c b/src/nvim/eval/typval.c
index 9a3a1c3c0f..ab4245b59a 100644
--- a/src/nvim/eval/typval.c
+++ b/src/nvim/eval/typval.c
@@ -654,14 +654,14 @@ tv_list_copy_error:
 /// @param[in] maxdepth   Maximum depth that will be flattened
 ///
 /// @return OK or FAIL
-int tv_list_flatten(list_T *list, long maxdepth)
-  FUNC_ATTR_NONNULL_ARG(1) FUNC_ATTR_WARN_UNUSED_RESULT
+void tv_list_flatten(list_T *list, long maxdepth)
+  FUNC_ATTR_NONNULL_ARG(1)
 {
   listitem_T *item;
   listitem_T *to_free;
   int n;
   if (maxdepth == 0) {
-    return OK;
+    return;
   }
 
   n = 0;
@@ -669,7 +669,7 @@ int tv_list_flatten(list_T *list, long maxdepth)
   while (item != NULL) {
     fast_breakcheck();
     if (got_int) {
-      return FAIL;
+      return;
     }
     if (item->li_tv.v_type == VAR_LIST) {
       listitem_T *next = item->li_next;
@@ -695,7 +695,6 @@ int tv_list_flatten(list_T *list, long maxdepth)
       item = item->li_next;
     }
   }
-  return OK;
 }
 
 /// Extend first list with the second
diff --git a/src/nvim/testdir/test_flatten.vim b/src/nvim/testdir/test_flatten.vim
index 99086611e1..d8abc1ad68 100644
--- a/src/nvim/testdir/test_flatten.vim
+++ b/src/nvim/testdir/test_flatten.vim
@@ -79,3 +79,14 @@ func Test_flatten()
   call assert_equal([1, 2, 1, 2], flatten(l:x, 2))
   call assert_equal([2, l:x], l:y)
 endfunc
+
+func Test_flattennew()
+  let l = [1, [2, [3, 4]], 5]
+  call assert_equal([1, 2, 3, 4, 5], flattennew(l))
+  call assert_equal([1, [2, [3, 4]], 5], l)
+
+  call assert_equal([1, 2, [3, 4], 5], flattennew(l, 1))
+  call assert_equal([1, [2, [3, 4]], 5], l)
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab