eval/typval: Make tv_list_concat handle NULL lists correctly

Fixes some FIXMEs in eval/null_spec.lua.

2 files changed, 19 insertions, 18 deletions

diff --git a/src/nvim/eval/typval.c b/src/nvim/eval/typval.c
index ae4199697f..9dd1b62d7d 100644
--- a/src/nvim/eval/typval.c
+++ b/src/nvim/eval/typval.c
@@ -498,20 +498,25 @@ void tv_list_extend(list_T *const l1, list_T *const l2,
 int tv_list_concat(list_T *const l1, list_T *const l2, typval_T *const tv)
   FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  if (l1 == NULL || l2 == NULL) {
-    return FAIL;
-  }
+  list_T *l;
 
-  // make a copy of the first list.
-  list_T *const l = tv_list_copy(NULL, l1, false, 0);
-  if (l == NULL) {
+  tv->v_type = VAR_LIST;
+
+  if (l1 == NULL && l2 == NULL) {
+    l = NULL;
+  } else if (l1 == NULL) {
+    l = tv_list_copy(NULL, l2, false, 0);
+  } else {
+    l = tv_list_copy(NULL, l1, false, 0);
+    if (l != NULL && l2 != NULL) {
+      tv_list_extend(l, l2, NULL);
+    }
+  }
+  if (l == NULL && !(l1 == NULL && l2 == NULL)) {
     return FAIL;
   }
-  tv->v_type = VAR_LIST;
-  tv->vval.v_list = l;
 
-  // append all items from the second list
-  tv_list_extend(l, l2, NULL);
+  tv->vval.v_list = l;
   return OK;
 }
 
diff --git a/test/functional/eval/null_spec.lua b/test/functional/eval/null_spec.lua
index 9a35905be0..ccdd1ce34c 100644
--- a/test/functional/eval/null_spec.lua
+++ b/test/functional/eval/null_spec.lua
@@ -61,14 +61,6 @@ describe('NULL', function()
     null_list_expr_test('is equal to itself', 'L == L', 0, 0)
     -- FIXME Should return 0
     null_list_expr_test('is not not equal to itself', 'L != L', 0, 1)
-    -- FIXME Should return empty list
-    null_list_expr_test('can be added to itself', '(L + L)', 'E15: Invalid expression: (L + L)', nil)
-    -- FIXME Should return [1]
-    null_list_expr_test('can be added to non-empty list (reversed)', '(L + [1])',
-                        'E15: Invalid expression: (L + [1])', nil)
-    -- FIXME Should return [1]
-    null_list_expr_test('can be added to non-empty list', '([1] + L)',
-                        'E15: Invalid expression: ([1] + L)', nil)
     -- FIXME Should return 1
     null_list_expr_test('counts correctly', 'count([L], L)', 0, 0)
     -- FIXME should be accepted by inputlist()
@@ -137,5 +129,9 @@ describe('NULL', function()
     null_list_expr_test('is stringified correctly', 'string(L)', 0, '[]')
     null_list_expr_test('is JSON encoded correctly', 'json_encode(L)', 0, '[]')
     null_list_test('does not crash lockvar', 'lockvar! L', 0)
+    null_list_expr_test('can be added to itself', '(L + L)', 0, {})
+    null_list_expr_test('can be added to itself', '(L + L) is L', 0, 1)
+    null_list_expr_test('can be added to non-empty list', '([1] + L)', 0, {1})
+    null_list_expr_test('can be added to non-empty list (reversed)', '(L + [1])', 0, {1})
   end)
 end)