glr: fix ambiguity reports.

Fix a regression introduced in commit
783aa653f4.

	* tests/glr-regression.at (Ambiguity reports): New.
	* data/glr.c (yyreportTree): Fix an offset error.

data/glr.c

@@ -1727,10 +1727,10 @@ yyreportTree (yySemanticOption* yyx, int yyindent)
         {
           if (yystates[yyi-1]->yyposn+1 > yystates[yyi]->yyposn)
             YYFPRINTF (stderr, "%*s%s <empty>\n", yyindent+2, "",
-                       yytokenName (yystos[yystates[yyi-1]->yylrState]));
+                       yytokenName (yystos[yystates[yyi]->yylrState]));
           else
             YYFPRINTF (stderr, "%*s%s <tokens %lu .. %lu>\n", yyindent+2, "",
-                       yytokenName (yystos[yystates[yyi-1]->yylrState]),
+                       yytokenName (yystos[yystates[yyi]->yylrState]),
                        (unsigned long int) (yystates[yyi-1]->yyposn + 1),
                        (unsigned long int) yystates[yyi]->yyposn);
         }

tests/glr-regression.at

@@ -1788,3 +1788,120 @@ glr-regr18.y:26.18-24: previous declaration
 ])
 
 AT_CLEANUP
+
+
+## ------------------- ##
+## Ambiguity reports.  ##
+## ------------------- ##
+
+AT_SETUP([Ambiguity reports])
+
+AT_DATA_GRAMMAR([input.y],
+[[
+%{
+  #include <stdio.h>
+  #include <stdlib.h>
+  static void yyerror (char const *);
+  static int yylex (void);
+%}
+
+%debug
+%glr-parser
+
+%%
+start:
+  'a' b 'c' d
+| 'a' b 'c' d
+;
+b: 'b';
+d: /* nada.  */;
+%%
+
+static int
+yylex (void)
+{
+  static char const input[] = "abc";
+  static size_t toknum;
+  if (! (toknum < sizeof input))
+    abort ();
+  return input[toknum++];
+}
+
+static void
+yyerror (char const *msg)
+{
+  fprintf (stderr, "%s\n", msg);
+}
+
+int
+main (void)
+{
+  yydebug = 1;
+  return !!yyparse ();
+}
+]])
+
+AT_BISON_CHECK([[-o input.c input.y]], 0, [],
+[input.y: conflicts: 1 reduce/reduce
+])
+AT_COMPILE([input])
+
+AT_PARSER_CHECK([[./input]], 1, [],
+[Starting parse
+Entering state 0
+Reading a token: Next token is token 'a' ()
+Shifting token 'a' ()
+Entering state 1
+Reading a token: Next token is token 'b' ()
+Shifting token 'b' ()
+Entering state 3
+Reducing stack 0 by rule 3 (line 25):
+   $1 = token 'b' ()
+-> $$ = nterm b ()
+Entering state 4
+Reading a token: Next token is token 'c' ()
+Shifting token 'c' ()
+Entering state 6
+Reducing stack 0 by rule 4 (line 26):
+-> $$ = nterm d ()
+Entering state 7
+Reading a token: Now at end of input.
+Stack 0 Entering state 7
+Now at end of input.
+Splitting off stack 1 from 0.
+Reduced stack 1 by rule #2; action deferred.  Now in state 2.
+Stack 1 Entering state 2
+Now at end of input.
+Reduced stack 0 by rule #1; action deferred.  Now in state 2.
+Merging stack 0 into stack 1.
+Stack 1 Entering state 2
+Now at end of input.
+Removing dead stacks.
+Rename stack 1 -> 0.
+On stack 0, shifting token $end ()
+Stack 0 now in state #5
+Ambiguity detected.
+Option 1,
+  start -> <Rule 1, tokens 1 .. 3>
+    'a' <tokens 1 .. 1>
+    b <tokens 2 .. 2>
+    'c' <tokens 3 .. 3>
+    d <empty>
+
+Option 2,
+  start -> <Rule 2, tokens 1 .. 3>
+    'a' <tokens 1 .. 1>
+    b <tokens 2 .. 2>
+    'c' <tokens 3 .. 3>
+    d <empty>
+
+syntax is ambiguous
+Cleanup: popping token $end ()
+Cleanup: popping unresolved nterm start ()
+Cleanup: popping nterm d ()
+Cleanup: popping token 'c' ()
+Cleanup: popping nterm b ()
+Cleanup: popping token 'a' ()
+])
+
+AT_CLEANUP