I use "bison" (GNU Bison 3.0.4) as the Linux kernel uses that.

From 78d27b370c61a65ede2c7a8d3bb4c130a729dff3 Mon Sep 17 00:00:00 2001
From: Bjarni Ingi Gislason <bjarniig@rhi.hi.is>
Date: Sat, 15 Sep 2018 22:17:13 +0000
Subject: [PATCH] pic.ypp: useless rule in parser due to conflicts

  Output from "make" with gcc-8.2.0:

  YACC     src/preproc/pic/pic.cpp
  YACC     src/preproc/refer/label.cpp
.../groff/src/preproc/pic/pic.ypp:1391.11-1396.17: warning: rule useless in parser due to conflicts [-Wother]
         | ORDINAL LAST object_type relative_path
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  The test seems to be of no concern and may thus be removed.
Otherwise, those who know what this means and is about, should replace
it with a correct test.

Signed-off-by: Bjarni Ingi Gislason <bjarniig@rhi.hi.is>
---
 src/preproc/pic/pic.ypp | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/src/preproc/pic/pic.ypp b/src/preproc/pic/pic.ypp
index 54ff44ab..a09b7ec2 100644
--- a/src/preproc/pic/pic.ypp
+++ b/src/preproc/pic/pic.ypp
@@ -1388,12 +1388,6 @@ path:
                   $$->set_ypath($4);
                 }
         /* The rest of these rules are a compatibility sop. */
-        | ORDINAL LAST object_type relative_path
-                {
-                  lex_warning("'%1%2 last %3' in 'with' argument ignored",
-                              $1, ordinal_postfix($1), object_type_name($3));
-                  $$ = $4;
-                }
         | LAST object_type relative_path
                 {
                   lex_warning("'last %1' in 'with' argument ignored",
--
2.18.0