fix(help): show error when using :help! with nothing at cursor #38775

It's possible to still show the old Easter egg, but then the user won't
know about the new feature, so showing E349 is better.

(cherry picked from commit 9705a1c13b)

src/nvim/errors.h

@@ -63,6 +63,7 @@ EXTERN const char e_nogroup[] INIT(= N_("E28: No such highlight group name: %s")
 EXTERN const char e_noinstext[] INIT(= N_("E29: No inserted text yet"));
 EXTERN const char e_nolastcmd[] INIT(= N_("E30: No previous command line"));
 EXTERN const char e_nomap[] INIT(= N_("E31: No such mapping"));
+EXTERN const char e_noident[] INIT(= N_("E349: No identifier under cursor"));
 EXTERN const char e_nomatch[] INIT(= N_("E479: No match"));
 EXTERN const char e_nomatch2[] INIT(= N_("E480: No match: %s"));
 EXTERN const char e_noname[] INIT(= N_("E32: No file name"));

src/nvim/help.c

@@ -114,6 +114,10 @@ void ex_help(exarg_T *eap)
     }
     api_free_object(res);
     api_clear_error(&err);
+    if (allocated_arg == NULL) {
+      emsg(_(e_noident));
+      return;
+    }
   }
 
   // Check if there is a match for the argument.

src/nvim/normal.c

@@ -132,8 +132,6 @@ static inline void normal_state_init(NormalState *s)
 // n_*(): functions called to handle Normal mode commands.
 // v_*(): functions called to handle Visual mode commands.
 
-static const char *e_noident = N_("E349: No identifier under cursor");
-
 /// Function to be called for a Normal or Visual mode command.
 /// The argument is a cmdarg_T.
 typedef void (*nv_func_T)(cmdarg_T *cap);

test/functional/ex_cmds/help_spec.lua

@@ -156,6 +156,10 @@ describe(':help', function()
     -- n.command [[set keywordprg=:help]]
 
     -- Failure modes:
+    set_lines ''
+    cursor(0, { 1, 1 })
+    t.matches('E349: No identifier under cursor', t.pcall_err(n.exec, [[:help!]]))
+
     set_lines 'xxxxxxxxx'
     cursor(0, { 1, 4 })
     t.matches('E149: No help for xxxxxxxxx', t.pcall_err(n.exec, [[:help!]]))