diagnostics: modernize bison's syntax errors

We used to display the unexpected token first:

    $ bison foo.y
    foo.y:1.8-13: error: syntax error, unexpected %token, expecting character literal or identifier or <tag>
        1 | %token %token
          |        ^~~~~~

GCC uses a different format:

    $ gcc-mp-9 foo.c
    foo.c:1:5: error: expected identifier or '(' before ')' token
        1 | int()()()
          |     ^

and so does Clang:

    $ clang-mp-9.0 foo.c
    foo.c:1:5: error: expected identifier or '('
    int()()()
        ^
    1 error generated.

They display the unexpected token last (or not at all).  Also, they
don't waste width with "syntax error".  Let's try that.  It gives, for
the same example as above:

    $ bison foo.y
    foo.y:1.8-13: error: expected character literal or identifier or <tag> before %token
        1 | %token %token
          |        ^~~~~~

* src/complain.h, src/complain.c (syntax_error): New.
* src/parse-gram.y (yyreport_syntax_error): Use it.

src/complain.c

@@ -23,6 +23,7 @@
 #include "system.h"
 
 #include <argmatch.h>
+#include <ctype.h>
 #include <progname.h>
 #include <stdarg.h>
 #include <sys/stat.h>
@@ -263,6 +264,20 @@ severity_prefix (severity s)
 }
 
 
+static void
+severity_print (severity s, FILE *out)
+{
+  if (s != severity_disabled)
+    {
+      const char* style = severity_style (s);
+      begin_use_class (style, out);
+      fprintf (out, "%s:", severity_prefix (s));
+      end_use_class (style, out);
+      fputc (' ', out);
+    }
+}
+
+
 /*-----------.
 | complain.  |
 `-----------*/
@@ -442,16 +457,7 @@ error_message (const location *loc, int *indent, warnings flags,
         fprintf (stderr, "%*s", *indent - pos, "");
     }
 
-  const char* style = severity_style (sever);
-
-  if (sever != severity_disabled)
-    {
-      begin_use_class (style, stderr);
-      fprintf (stderr, "%s:", severity_prefix (sever));
-      end_use_class (style, stderr);
-      fputc (' ', stderr);
-    }
-
+  severity_print (sever, stderr);
   vfprintf (stderr, message, args);
   /* Print the type of warning, only if this is not a sub message
      (in which case the prefix is null).  */
@@ -465,7 +471,7 @@ error_message (const location *loc, int *indent, warnings flags,
         putc ('\n', stderr);
         flush (stderr);
         if (loc && !(flags & no_caret))
-          location_caret (*loc, style, stderr);
+          location_caret (*loc, severity_style (sever), stderr);
       }
   }
   flush (stderr);
@@ -587,3 +593,53 @@ duplicate_rule_directive (char const *directive,
                    _("previous declaration"));
   fixits_register (&second, "");
 }
+
+void
+syntax_error (location loc,
+              int argc, const char* argv[])
+{
+  if (complaint_status < status_complaint)
+    complaint_status = status_complaint;
+  assert (argc <= 5);
+  const char *format = NULL;
+  switch (argc)
+    {
+# define CASE(N, S)                         \
+      case N:                               \
+        format = S;                         \
+        break
+    default: /* Avoid compiler warnings. */
+      CASE (0, _("syntax error"));
+      CASE (1, _("unexpected %0$s"));
+      CASE (2, _("expected %1$s before %0$s"));
+      CASE (3, _("expected %1$s or %2$s before %0$s"));
+      CASE (4, _("expected %1$s or %2$s or %3$s before %0$s"));
+      CASE (5, _("expected %1$s or %2$s or %4$s or %5$s before %0$s"));
+# undef CASE
+    }
+  location_print (loc, stderr);
+  fputs (": ", stderr);
+  severity_print (severity_error, stderr);
+
+  while (*format)
+    if (format[0] == '%'
+        && isdigit (format[1])
+        && format[2] == '$'
+        && format[3] == 's'
+        && (format[1] - '0') < argc)
+      {
+        int i = format[1] - '0';
+        const char *style = i == 0 ? "unexpected" : "expected";
+        begin_use_class (style, stderr);
+        fputs (argv[i], stderr);
+        end_use_class (style, stderr);
+        format += 4;
+      }
+    else
+      {
+        fputc (*format, stderr);
+        ++format;
+      }
+  fputc ('\n', stderr);
+  location_caret (loc, "error", stderr);
+}

src/complain.h

@@ -161,6 +161,11 @@ void duplicate_directive (char const *directive,
 void duplicate_rule_directive (char const *directive,
                                location first, location second);
 
+/** Report a syntax error, where argv[0] is the unexpected
+    token, and argv[1...argc] are the expected ones.  */
+void syntax_error (location loc,
+                   int argc, const char* argv[]);
+
 /** Warnings treated as errors shouldn't stop the execution as regular
     errors should (because due to their nature, it is safe to go
     on). Thus, there are three possible execution statuses.  */

src/parse-gram.y

@@ -804,58 +804,17 @@ epilogue.opt:
 int
 yyreport_syntax_error (const yyparse_context_t *ctx)
 {
-  if (complaint_status < status_complaint)
-    complaint_status = status_complaint;
   enum { YYERROR_VERBOSE_ARGS_MAXIMUM = 5 };
-  /* Internationalized format string. */
-  const char *format = YY_NULLPTR;
   /* Arguments of format: reported tokens (one for the "unexpected",
      one per "expected"). */
   int arg[YYERROR_VERBOSE_ARGS_MAXIMUM];
   int n = yysyntax_error_arguments (ctx, arg, YYERROR_VERBOSE_ARGS_MAXIMUM);
-  switch (n)
-    {
-    case -2:
-      return 2;
-# define YYCASE_(N, S)                      \
-      case N:                               \
-        format = S;                         \
-      break
-    default: /* Avoid compiler warnings. */
-      YYCASE_(0, YY_("syntax error"));
-      YYCASE_(1, YY_("syntax error, unexpected %s"));
-      YYCASE_(2, YY_("syntax error, unexpected %s, expecting %s"));
-      YYCASE_(3, YY_("syntax error, unexpected %s, expecting %s or %s"));
-      YYCASE_(4, YY_("syntax error, unexpected %s, expecting %s or %s or %s"));
-      YYCASE_(5, YY_("syntax error, unexpected %s, expecting %s or %s or %s or %s"));
-# undef YYCASE_
-    }
-  location_print (*yyparse_context_location (ctx), stderr);
-  fputs (": ", stderr);
-  begin_use_class ("error", stderr);
-  fputs ("error:", stderr);
-  end_use_class ("error", stderr);
-  fputc (' ', stderr);
-  {
-    int i = 0;
-    while (*format)
-      if (format[0] == '%' && format[1] == 's' && i < n)
-        {
-          const char *style = i == 0 ? "unexpected" : "expected";
-          begin_use_class (style, stderr);
-          fputs (yysymbol_name (arg[i]), stderr);
-          end_use_class (style, stderr);
-          format += 2;
-          ++i;
-        }
-      else
-        {
-          fputc (*format, stderr);
-          ++format;
-        }
-  }
-  fputc ('\n', stderr);
-  location_caret (*yyparse_context_location (ctx), "error", stderr);
+  if (n == -2)
+    return 2;
+  const char *argv[YYERROR_VERBOSE_ARGS_MAXIMUM];
+  for (int i = 0; i < n; ++i)
+    argv[i] = yysymbol_name (arg[i]);
+  syntax_error (*yyparse_context_location (ctx), n, argv);
   return 0;
 }