patch 8.2.1949: Vim9: using extend() on null dict is silently ignored

Problem:    Vim9: using extend() on null dict is silently ignored.
Solution:   Give an error message.  Initialize a dict variable with an empty
            dictionary. (closes #7251)

src/errors.h

@@ -288,4 +288,8 @@ EXTERN char e_cannot_add_to_null_blob[]
 	INIT(= N_("E1131: Cannot add to null blob"));
 EXTERN char e_missing_function_argument[]
 	INIT(= N_("E1132: Missing function argument"));
+EXTERN char e_cannot_extend_null_dict[]
+	INIT(= N_("E1133: Cannot extend a null dict"));
+EXTERN char e_cannot_extend_null_list[]
+	INIT(= N_("E1134: Cannot extend a null list"));
 #endif

src/evalvars.c

@@ -2553,7 +2553,22 @@ eval_variable(
 	    ret = FAIL;
 	}
 	else if (rettv != NULL)
+	{
+	    // If a list or dict variable wasn't initialized, do it now.
+	    if (tv->v_type == VAR_DICT && tv->vval.v_dict == NULL)
+	    {
+		tv->vval.v_dict = dict_alloc();
+		if (tv->vval.v_dict != NULL)
+		    ++tv->vval.v_dict->dv_refcount;
+	    }
+	    else if (tv->v_type == VAR_LIST && tv->vval.v_list == NULL)
+	    {
+		tv->vval.v_list = list_alloc();
+		if (tv->vval.v_list != NULL)
+		    ++tv->vval.v_list->lv_refcount;
+	    }
 	    copy_tv(tv, rettv);
+	}
     }
 
     name[len] = cc;

src/list.c

@@ -2303,9 +2303,13 @@ f_extend(typval_T *argvars, typval_T *rettv)
 	int		error = FALSE;
 
 	l1 = argvars[0].vval.v_list;
+	if (l1 == NULL)
+	{
+	    emsg(_(e_cannot_extend_null_list));
+	    return;
+	}
 	l2 = argvars[1].vval.v_list;
-	if (l1 != NULL && !value_check_lock(l1->lv_lock, arg_errmsg, TRUE)
-		&& l2 != NULL)
+	if (!value_check_lock(l1->lv_lock, arg_errmsg, TRUE) && l2 != NULL)
 	{
 	    if (argvars[2].v_type != VAR_UNKNOWN)
 	    {
@@ -2339,9 +2343,13 @@ f_extend(typval_T *argvars, typval_T *rettv)
 	int	i;
 
 	d1 = argvars[0].vval.v_dict;
+	if (d1 == NULL)
+	{
+	    emsg(_(e_cannot_extend_null_dict));
+	    return;
+	}
 	d2 = argvars[1].vval.v_dict;
-	if (d1 != NULL && !value_check_lock(d1->dv_lock, arg_errmsg, TRUE)
-		&& d2 != NULL)
+	if (!value_check_lock(d1->dv_lock, arg_errmsg, TRUE) && d2 != NULL)
 	{
 	    // Check the third argument.
 	    if (argvars[2].v_type != VAR_UNKNOWN)

src/testdir/test_vim9_assign.vim

@@ -231,10 +231,14 @@ def Test_extend_list()
       var l: list<number>
       l += [123]
       assert_equal([123], l)
+  END
+  CheckScriptSuccess(lines)
 
-      var d: dict<number>
-      d['one'] = 1
-      assert_equal(#{one: 1}, d)
+  lines =<< trim END
+      vim9script
+      var list: list<string>
+      extend(list, ['x'])
+      assert_equal(['x'], list)
   END
   CheckScriptSuccess(lines)
 
@@ -249,6 +253,48 @@ def Test_extend_list()
       assert_equal(['a', 'b'], list)
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      var l: list<string> = test_null_list()
+      extend(l, ['x'])
+      assert_equal(['x'], l)
+  END
+  CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      extend(test_null_list(), ['x'])
+  END
+  CheckScriptFailure(lines, 'E1134:', 2)
+enddef
+
+def Test_extend_dict()
+  var lines =<< trim END
+      vim9script
+      var d: dict<number>
+      extend(d, #{a: 1})
+      assert_equal(#{a: 1}, d)
+
+      var d2: dict<number>
+      d2['one'] = 1
+      assert_equal(#{one: 1}, d2)
+  END
+  CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      var d: dict<string> = test_null_dict()
+      extend(d, #{a: 'x'})
+      assert_equal(#{a: 'x'}, d)
+  END
+  CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      extend(test_null_dict(), #{a: 'x'})
+  END
+  CheckScriptFailure(lines, 'E1133:', 2)
 enddef
 
 def Test_single_letter_vars()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1949,
 /**/
     1948,
 /**/