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);
+  severity_print (sever, stderr);
-
-  if (sever != severity_disabled)
-    {
-      begin_use_class (style, stderr);
-      fprintf (stderr, "%s:", severity_prefix (sever));
-      end_use_class (style, stderr);
-      fputc (' ', 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)
+  if (n == -2)
-    {
+    return 2;
-    case -2:
+  const char *argv[YYERROR_VERBOSE_ARGS_MAXIMUM];
-      return 2;
+  for (int i = 0; i < n; ++i)
-# define YYCASE_(N, S)                      \
+    argv[i] = yysymbol_name (arg[i]);
-      case N:                               \
+  syntax_error (*yyparse_context_location (ctx), n, argv);
-        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);
   return 0;
 }
 

tests/diagnostics.at

@@ -273,7 +273,7 @@ AT_TEST([[Carriage return]],
 [[input.y:10.8-11.0: <error>error:</error> missing '"' at end of line
    10 | %token <error>"</error>
       |        <error>^</error>
-input.y:10.8-11.0: <error>error:</error> syntax error, unexpected <unexpected>string</unexpected>, expecting <expected>character literal</expected> or <expected>identifier</expected> or <expected><tag></expected>
+input.y:10.8-11.0: <error>error:</error> expected <expected>character literal</expected> or <expected>identifier</expected> or <expected><tag></expected> before <unexpected>string</unexpected>
    10 | %token <error>"</error>
       |        <error>^</error>
 ]])

tests/input.at

@@ -102,7 +102,7 @@ input.y:6.1-17: error: invalid directive: '%a-does-not-exist'
 input.y:7.1: error: invalid character: '%'
 input.y:7.2: error: invalid character: '-'
 input.y:8.1-9.0: error: missing '%}' at end of file
-input.y:8.1-9.0: error: syntax error, unexpected %{...%}
+input.y:8.1-9.0: error: unexpected %{...%}
 ]])
 
 AT_CLEANUP
@@ -124,7 +124,7 @@ AT_DATA([input.y],
 ]])
 
 AT_BISON_CHECK([input.y], [1], [],
-[[input.y:3.1-15: error: syntax error, unexpected %initial-action, expecting {...}
+[[input.y:3.1-15: error: expected {...} before %initial-action
 ]])
 
 AT_CLEANUP
@@ -281,16 +281,16 @@ input.y:3.17-24: error: nonterminals cannot be given a string alias
 input.y:4.8-10: error: character literals cannot be nonterminals
     4 | %nterm '+' '*';
       |        ^~~
-input.y:5.8-15: error: syntax error, unexpected string, expecting character literal or identifier or <tag>
+input.y:5.8-15: error: expected character literal or identifier or <tag> before string
     5 | %nterm "number";
       |        ^~~~~~~~
-input.y:6.8-13: error: syntax error, unexpected string, expecting character literal or identifier or <tag>
+input.y:6.8-13: error: expected character literal or identifier or <tag> before string
     6 | %token "tok1" 1;
       |        ^~~~~~
-input.y:7.14: error: syntax error, unexpected integer literal
+input.y:7.14: error: unexpected integer literal
     7 | %left "tok2" 2;
       |              ^
-input.y:8.14: error: syntax error, unexpected integer literal
+input.y:8.14: error: unexpected integer literal
     8 | %type "tok3" 3;
       |              ^
 ]])
@@ -1277,7 +1277,7 @@ AT_SETUP([Torturing the Scanner])
 AT_BISON_OPTION_PUSHDEFS
 AT_DATA([input.y], [])
 AT_BISON_CHECK([input.y], [1], [],
-[[input.y:1.1: error: syntax error, unexpected end of file
+[[input.y:1.1: error: unexpected end of file
 ]])
 
 
@@ -1285,7 +1285,7 @@ AT_DATA([input.y],
 [{}
 ])
 AT_BISON_CHECK([-fcaret  input.y], [1], [],
-[[input.y:1.1-2: error: syntax error, unexpected {...}
+[[input.y:1.1-2: error: unexpected {...}
     1 | {}
       | ^~
 ]])
@@ -1655,7 +1655,7 @@ input.y:16.11-17.0: error: missing '"' at end of line
 input.y:19.13-20.0: error: missing '}' at end of file
    19 | %destructor { free ($$)
       |             ^~~~~~~~~~~
-input.y:20.1: error: syntax error, unexpected end of file
+input.y:20.1: error: unexpected end of file
 ]])
 
 AT_CLEANUP