%nterm: do not accept character literals

Reported by Rici Lake.
http://lists.gnu.org/archive/html/bug-bison/2018-10/msg00000.html

* src/complain.h: Formatting change.
* src/parse-gram.y (id): Reject character literals used in a context
for non-terminals.
* tests/input.at (Invalid %nterm uses): Check that.

src/complain.h

@@ -78,8 +78,7 @@ void complain_init (void);
 
 typedef enum
   {
-    /**< Issue no warnings.  */
-    Wnone             = 0,
+    Wnone             = 0,       /**< Issue no warnings.  */
 
     Wmidrule_values   = 1 << warning_midrule_values,
     Wyacc             = 1 << warning_yacc,

src/parse-gram.y

@@ -553,6 +553,9 @@ symbol_def:
 symbol_defs.1:
   symbol_def
 | symbol_defs.1 symbol_def
+  /* FIXME: cannot do that, results in infinite loop in LAC.
+| error                    { yyerrok; }
+  */
 ;
 
 
@@ -669,6 +672,12 @@ id:
     { $$ = symbol_from_uniqstr ($1, @1); }
 | CHAR
     {
+      if (current_class == nterm_sym)
+        {
+          gram_error (&@1,
+                      _("character literals cannot be non-terminals"));
+          YYERROR;
+        }
       $$ = symbol_get (char_name ($1), @1);
       symbol_class_set ($$, token_sym, @1, false);
       symbol_user_token_number_set ($$, $1, @1);