eval: Forbid (un)locking b:changedtick

Port of vim-patch:8.0.0343

3 files changed, 18 insertions, 7 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 1105601a0e..88ab502ab6 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -8606,9 +8606,12 @@ This does NOT work: >
 				:lockvar v
 				:let v = 'asdf'		" fails!
 				:unlet v
-<							*E741*
+<							*E741* *E940*
 			If you try to change a locked variable you get an
-			error message: "E741: Value is locked: {name}"
+			error message: "E741: Value is locked: {name}".
+			If you try to lock or unlock a built-in variable you 
+			will get an error message "E940: Cannot lock or unlock 
+			variable {name}".
 
 			[depth] is relevant when locking a |List| or
 			|Dictionary|.  It specifies how deep the locking goes:
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index da084d8ecc..25ac0e2ad1 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -3091,6 +3091,12 @@ static int do_lock_var(lval_T *lp, char_u *name_end, int deep, int lock)
         true);
     if (di == NULL) {
       ret = FAIL;
+    } else if ((di->di_flags & DI_FLAGS_FIX)
+               && di->di_tv.v_type != VAR_DICT
+               && di->di_tv.v_type != VAR_LIST) {
+      // For historical reasons this error is not given for Lists and
+      // Dictionaries. E.g. b: dictionary may be locked/unlocked.
+      emsgf(_("E940: Cannot lock or unlock variable %s"), lp->ll_name);
     } else {
       if ((di->di_flags & (DI_FLAGS_LOCK|DI_FLAGS_FIX))
           == (DI_FLAGS_LOCK|DI_FLAGS_FIX)) {
diff --git a/test/functional/eval/changedtick_spec.lua b/test/functional/eval/changedtick_spec.lua
index 83b07e6722..74e581bd3c 100644
--- a/test/functional/eval/changedtick_spec.lua
+++ b/test/functional/eval/changedtick_spec.lua
@@ -101,19 +101,21 @@ describe('b:changedtick', function()
        redir_exec(':let b:'))
   end)
   it('fails to unlock b:changedtick', function()
-    -- Note:
-    -- - unlocking VAR_FIXED variables is not an error.
-    -- - neither VAR_FIXED variables are reported as locked by islocked().
-    -- So test mostly checks that b:changedtick status does not change.
     eq(0, exc_exec('let d = b:'))
     eq(0, funcs.islocked('b:changedtick'))
     eq(0, funcs.islocked('d.changedtick'))
-    eq('',
+    eq('\nE940: Cannot lock or unlock variable b:changedtick',
        redir_exec('unlockvar b:changedtick'))
     eq('\nE46: Cannot change read-only variable "d.changedtick"',
        redir_exec('unlockvar d.changedtick'))
     eq(0, funcs.islocked('b:changedtick'))
     eq(0, funcs.islocked('d.changedtick'))
+    eq('\nE940: Cannot lock or unlock variable b:changedtick',
+       redir_exec('lockvar b:changedtick'))
+    eq('\nE46: Cannot change read-only variable "d.changedtick"',
+       redir_exec('lockvar d.changedtick'))
+    eq(0, funcs.islocked('b:changedtick'))
+    eq(0, funcs.islocked('d.changedtick'))
   end)
   it('is being completed', function()
     feed(':echo b:<Tab><Home>let cmdline="<End>"<CR>')