[PATCH 5/9] yacc.c: also define a symbol number for the empty token

[Akim Demaille]

This is not only cleaner, it also protects us from mixing signed
values (YYEMPTY is #defined as -2) with unsigned types (the
yysymbol_code_t enum is typically compiled as a small unsigned).
For instance GCC 9:

    input.c: In function 'yyparse':
    input.c:1107:7: error: conversion to 'unsigned int' from 'int'
                           may change the sign of the result
                           [-Werror=sign-conversion]
     1107 |   yyn += yytoken;
          |       ^~
    input.c:1107:10: error: conversion to 'int' from 'unsigned int'
                            may change the sign of the result
                            [-Werror=sign-conversion]
     1107 |   yyn += yytoken;
          |          ^~~~~~~
    input.c:1108:47: error: comparison of integer expressions of
                            different signedness:
                            'yytype_int8' {aka 'const signed char'} and
                            'yysymbol_code_t' {aka 'enum yysymbol_code_t'}
                            [-Werror=sign-compare]
     1108 |   if (yyn < 0 || YYLAST < yyn || yycheck[yyn] != yytoken)
          |                                               ^~
    input.c:702:25: error: operand of ?: changes signedness from 'int'
                           to 'unsigned int' due to unsignedness of
                           other operand [-Werror=sign-compare]
      702 | #define YYEMPTY         (-2)
          |                         ^~~~
    input.c:1220:33: note: in expansion of macro 'YYEMPTY'
     1220 |   yytoken = yychar == YYEMPTY ? YYEMPTY : YYTRANSLATE (yychar);
          |                                 ^~~~~~~
    input.c:1220:41: error: unsigned conversion from 'int' to
                            'unsigned int' changes value
                            from '-2' to '4294967294'
                            [-Werror=sign-conversion]
     1220 |   yytoken = yychar == YYEMPTY ? YYEMPTY : YYTRANSLATE (yychar);
          |                                         ^

Eventually, it might be interesting to move away from -2 (which is the
only possible negative symbol number) and use the next available
number, to save bits.  We could actually even simply use "0" and shift
the rest, which would allow to write "!yytoken" to mean really
"yytoken != YYEMPTY".

* data/skeletons/c.m4 (b4_declare_symbol_enum): Define YYSYMBOL_YYEMPTY.
* data/skeletons/yacc.c: Use it.

* tests/regression.at: Adjust.
---
 TODO                    | 16 ++++++++++++++++
 data/skeletons/bison.m4 |  1 +
 data/skeletons/c.m4     |  3 +++
 data/skeletons/yacc.c   |  4 ++--
 tests/regression.at     |  2 +-
 5 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/TODO b/TODO
index 2489f7a4..0913481a 100644
--- a/TODO
+++ b/TODO
@@ -52,6 +52,22 @@ would actually also make the following point gracefully 
handled (status of
 YYERRCODE, YYUNDEFTOK, etc.).  Possibly we could also define YYEMPTY (twice:
 as a token and as a symbol).  And YYEOF.
 
+It seems to work well.  Yet we have a weird case: the "error" token:
+
+enum yysymbol_code_t
+{
+  YYSYMBOL_YYEMPTY = -2,
+  YYSYMBOL_YYEOF = 0,
+  YYSYMBOL_error = 1,
+  YYSYMBOL_YYUNDEF = 2,
+  YYSYMBOL_YYACCEPT = 61,
+  ...
+
+YYSYMBOL_error looks weird.  We should maybe rename this as
+"YYSYMBOL_YYERROR", even though it should not be confonded with the YYERROR
+macro.
+
+
 ** Consistency
 YYUNDEFTOK is an internal symbol number, as YYTERROR.
 But YYERRCODE is an external token number.
diff --git a/data/skeletons/bison.m4 b/data/skeletons/bison.m4
index ab4fb720..20de1b7e 100644
--- a/data/skeletons/bison.m4
+++ b/data/skeletons/bison.m4
@@ -411,6 +411,7 @@ m4_define([_b4_symbol],
 # if that would produce an invalid symbol.
 m4_define([b4_symbol_sid],
 [m4_case([$1],
+  [-2],                           [[YYSYMBOL_YYEMPTY]],
   [0],                            [[YYSYMBOL_YYEOF]],
   [m4_bmatch(m4_quote(b4_symbol([$1], [tag])),
       [^\$accept$],               [[YYSYMBOL_YYACCEPT]],
diff --git a/data/skeletons/c.m4 b/data/skeletons/c.m4
index d9abaf13..659e85b9 100644
--- a/data/skeletons/c.m4
+++ b/data/skeletons/c.m4
@@ -502,12 +502,15 @@ m4_define([b4_symbol_enum],
 # b4_declare_symbol_enum
 # ----------------------
 # The definition of the symbol internal numbers as an enum.
+# Defining YYEMPTY here is important: it forces the compiler
+# to use a signed type, which matters for yytoken.
 m4_define([b4_declare_symbol_enum],
 [[/* Symbol type.  */
 enum yysymbol_code_t
 {
   ]m4_join([,
   ],
+           ]b4_symbol_sid([-2])[ = -2,
            b4_symbol_map([b4_symbol_enum]))[
 };
 typedef enum yysymbol_code_t yysymbol_code_t;
diff --git a/data/skeletons/yacc.c b/data/skeletons/yacc.c
index ab1a8890..2ee1fe3f 100644
--- a/data/skeletons/yacc.c
+++ b/data/skeletons/yacc.c
@@ -1348,7 +1348,7 @@ yysyntax_error_arguments (const yyparse_context_t *yyctx,
        one exception: it will still contain any token that will not be
        accepted due to an error action in a later state.]])[
   */
-  if (yyctx->yytoken != YYEMPTY)
+  if (yyctx->yytoken != YYSYMBOL_YYEMPTY)
     {
       int yyn;]b4_lac_if([[
       YYDPRINTF ((stderr, "Constructing syntax error message\n"));]])[
@@ -1889,7 +1889,7 @@ yyreduce:
 yyerrlab:
   /* Make sure we have latest lookahead translation.  See comments at
      user semantic actions for why this is necessary.  */
-  yytoken = yychar == YYEMPTY ? YYEMPTY : YYTRANSLATE (yychar);
+  yytoken = yychar == YYEMPTY ? YYSYMBOL_YYEMPTY : YYTRANSLATE (yychar);
 
   /* If not already recovering from an error, report this error.  */
   if (!yyerrstatus)
diff --git a/tests/regression.at b/tests/regression.at
index cd79b507..f20c7111 100644
--- a/tests/regression.at
+++ b/tests/regression.at
@@ -665,7 +665,7 @@ AT_BISON_CHECK([-v -o input.c input.y])
 [sed -n 's/  *$//;/^static const.*\[\] =/,/^}/p' input.c >tables.c]
 
 AT_CHECK([[cat tables.c]], 0,
-[[static const yytype_int8 yytranslate[] =
+[[static const yysymbol_code_t yytranslate[] =
 {
        0,     2,     2,     2,     2,     2,     2,     2,     2,     2,
        2,     2,     2,     2,     2,     2,     2,     2,     2,     2,
-- 
2.25.1