patch 8.2.4492: no error if an option is given a value with ":let &op…

…t = val"

Problem:    No error if an option is given an invalid value with
            ":let &opt = val".
Solution:   Give the error. (closes #9864)

src/evalvars.c

@@ -1518,8 +1518,11 @@ ex_let_option(
 	{
 	    if (opt_type != gov_string || s != NULL)
 	    {
-		set_option_value(arg, n, s, scope);
+		char *err = set_option_value(arg, n, s, scope);
+
 		arg_end = p;
+		if (err != NULL)
+		    emsg(_(err));
 	    }
 	    else
 		emsg(_(e_string_required));

src/testdir/test_options.vim

@@ -369,6 +369,7 @@ func Test_set_errors()
   call assert_fails('set sidescroll=-1', 'E487:')
   call assert_fails('set tabstop=-1', 'E487:')
   call assert_fails('set tabstop=10000', 'E474:')
+  call assert_fails('let &tabstop = 10000', 'E474:')
   call assert_fails('set tabstop=5500000000', 'E474:')
   call assert_fails('set textwidth=-1', 'E487:')
   call assert_fails('set timeoutlen=-1', 'E487:')
@@ -384,6 +385,7 @@ func Test_set_errors()
   call assert_fails('set comments=a', 'E525:')
   call assert_fails('set foldmarker=x', 'E536:')
   call assert_fails('set commentstring=x', 'E537:')
+  call assert_fails('let &commentstring = "x"', 'E537:')
   call assert_fails('set complete=x', 'E539:')
   call assert_fails('set rulerformat=%-', 'E539:')
   call assert_fails('set rulerformat=%(', 'E542:')

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4492,
 /**/
     4491,
 /**/