yacc.c: also define a symbol number for the empty token

This is not only cleaner, it also protects us from mixing signed
values (YYEMPTY is #defined as -2) with unsigned types (the
yysymbol_type_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_type_t' {aka 'enum yysymbol_type_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.

* src/parse-gram.y (yyreport_syntax_error): Use YYSYMBOL_YYEMPTY, not
YYEMPTY, when dealing with a symbol.

* tests/regression.at: Adjust.

TODO

@@ -53,6 +53,22 @@ 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_type_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.

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]],

data/skeletons/c.m4

@@ -504,12 +504,15 @@ 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_type_t
 {
   ]m4_join([,
   ],
+           ]b4_symbol_sid([-2])[ = -2,
            b4_symbol_map([b4_symbol_enum]))[
 };
 typedef enum yysymbol_type_t yysymbol_type_t;

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"));]])[
@@ -1576,7 +1576,7 @@ yyparse (]m4_ifset([b4_parse_param], [b4_formals(b4_parse_param)], [void])[)]])[
   /* The return value of yyparse.  */
   int yyresult;
   /* Lookahead token as an internal (translated) token number.  */
-  yysymbol_type_t yytoken = 0;
+  yysymbol_type_t yytoken = YYSYMBOL_YYEMPTY;
   /* The variables used to return semantic value and location from the
      action routines.  */
   YYSTYPE yyval;]b4_locations_if([[
@@ -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)

src/parse-gram.y

@@ -808,7 +808,7 @@ yyreport_syntax_error (const yyparse_context_t *ctx)
   const char *argv[ARGS_MAX];
   int argc = 0;
   yysymbol_type_t unexpected = yyparse_context_token (ctx);
-  if (unexpected != YYEMPTY)
+  if (unexpected != YYSYMBOL_YYEMPTY)
     {
       argv[argc++] = yysymbol_name (unexpected);
       yysymbol_type_t expected[ARGS_MAX - 1];

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_type_t yytranslate[] =
 {
        0,     2,     2,     2,     2,     2,     2,     2,     2,     2,
        2,     2,     2,     2,     2,     2,     2,     2,     2,     2,