patch 8.2.4505: Vim9: outdated "autocmd nested" still works

Problem:    Vim9: outdated "autocmd nested" still works.
Solution:   Do not accept the :autocmd argument "nested" without "++" in Vim9
            script.

src/autocmd.c

@@ -921,16 +921,30 @@ do_autocmd(exarg_T *eap, char_u *arg_in, int forceit)
 		if ((STRNCMP(cmd, "++nested", 8) == 0 && VIM_ISWHITE(cmd[8])))
 		{
 		    if (nested)
+		    {
 			semsg(_(e_duplicate_argument_str), "++nested");
+			return;
+		    }
 		    nested = TRUE;
 		    cmd = skipwhite(cmd + 8);
 		}
 
-		// Check for the old "nested" flag.
+		// Check for the old "nested" flag in legacy script.
 		if (STRNCMP(cmd, "nested", 6) == 0 && VIM_ISWHITE(cmd[6]))
 		{
+		    if (in_vim9script())
+		    {
+			// If there ever is a :nested command this error should
+			// be removed and "nested" accepted as the start of the
+			// command.
+			emsg(_(e_invalid_command_nested_did_you_mean_plusplus_nested));
+			return;
+		    }
 		    if (nested)
+		    {
 			semsg(_(e_duplicate_argument_str), "nested");
+			return;
+		    }
 		    nested = TRUE;
 		    cmd = skipwhite(cmd + 6);
 		}

src/errors.h

@@ -2786,7 +2786,10 @@ EXTERN char e_this_vim_is_not_compiled_with_float_support[]
 # endif
 EXTERN char e_missing_argument_type_for_str[]
 	INIT(= N_("E1077: Missing argument type for %s"));
-// E1078 unused
+#endif
+EXTERN char e_invalid_command_nested_did_you_mean_plusplus_nested[]
+	INIT(= N_("E1078: Invalid command \"nested\", did you mean \"++nested\"?"));
+#ifdef FEAT_EVAL
 EXTERN char e_cannot_declare_variable_on_command_line[]
 	INIT(= N_("E1079: Cannot declare a variable on the command line"));
 EXTERN char e_invalid_assignment[]

src/testdir/test_autocmd.vim

@@ -2116,6 +2116,9 @@ func Test_autocmd_nested()
   close
   bwipe! somefile
 
+  " nested without ++ does not work in Vim9 script
+  call assert_fails('vim9cmd au WinNew * nested echo fails', 'E1078:')
+
   augroup Testing
     au!
   augroup END

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4505,
 /**/
     4504,
 /**/