[PATCH] lalr1.cc: don't discard non-existent lookahead on syntax error.

[Joel E. Denny]

I pushed this to master.  The bug does not exist on branch-2.5.  The test 
case makes this bug look pretty obscure, and maybe that's right, but I 
really didn't spend much time searching for a simpler test case.

>From d59beda068aef97834ea84ce4f5cc99b487e4b68 Mon Sep 17 00:00:00 2001
From: Joel E. Denny <address@hidden>
Date: Fri, 18 Dec 2009 23:57:18 -0500
Subject: [PATCH] lalr1.cc: don't discard non-existent lookahead on syntax error.

* data/lalr1.cc (parser::parse): Check yyempty first.
* tests/c++.at (Syntax error discarding no lookahead): New test
group.
---
 ChangeLog     |    7 +++++
 data/lalr1.cc |    2 +-
 tests/c++.at  |   79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 85 insertions(+), 3 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 2926da7..1c15cc5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2009-12-18  Joel E. Denny  <address@hidden>
+
+       lalr1.cc: don't discard non-existent lookahead on syntax error.
+       * data/lalr1.cc (parser::parse): Check yyempty first.
+       * tests/c++.at (Syntax error discarding no lookahead): New test
+       group.
+
 2009-12-17  Joel E. Denny  <address@hidden>
 
        Code cleanup.
diff --git a/data/lalr1.cc b/data/lalr1.cc
index 9e0d522..6174e82 100644
--- a/data/lalr1.cc
+++ b/data/lalr1.cc
@@ -876,7 +876,7 @@ m4_ifdef([b4_lex_param], [, ]b4_lex_param)));])[
         /* Return failure if at end of input.  */
         if (yyla.type == yyeof_)
           YYABORT;
-        else
+        else if (!yyempty)
           {
             yy_destroy_ ("Error: discarding", yyla);
             yyempty = true;
diff --git a/tests/c++.at b/tests/c++.at
index beffb1c..ce64d6d 100644
--- a/tests/c++.at
+++ b/tests/c++.at
@@ -286,8 +286,6 @@ AT_CHECK_DOXYGEN([Public])
 AT_CHECK_DOXYGEN([Private])
 
 
-
-
 ## ------------ ##
 ## Namespaces.  ##
 ## ------------ ##
@@ -369,3 +367,80 @@ AT_CHECK_NAMESPACE([[foo[3]::bar::baz]], [[-]])
 AT_CHECK_NAMESPACE([[foo::bar,baz]], [[-]])
 AT_CHECK_NAMESPACE([[foo::bar::(baz /* Pacify Emacs ) */]], [[-]])
 AT_CLEANUP
+
+
+## -------------------------------------- ##
+## Syntax error discarding no lookahead.  ##
+## -------------------------------------- ##
+
+# After a syntax error, lalr1.cc used to not check whether there
+# actually is a lookahead before discarding the lookahead.  As a result,
+# it mistakenly invoked the destructor for the previous lookahead.
+
+AT_SETUP([[Syntax error discarding no lookahead]])
+
+AT_DATA_GRAMMAR([[input.yy]],
+[[%skeleton "lalr1.cc"
+
+%code {
+  #include <string>
+  int yylex (yy::parser::semantic_type *, yy::location *);
+  #define USE(Args)
+}
+
+%defines
+%locations
+%define parse.error verbose
+
+%nonassoc 'a' ;
+
+%destructor {
+  std::cerr << "Discarding 'a'." << std::endl;
+} 'a'
+
+%%
+
+start: error-reduce consistent-error 'a' { USE ($3); };
+
+error-reduce:
+  'a' 'a' consistent-error 'a' { USE (($1, $2, $4)); }
+| 'a' error { std::cerr << "Reducing 'a'." << std::endl; USE ($1); }
+;
+
+consistent-error:
+  'a'
+| /*empty*/ %prec 'a'
+;
+
+%%
+
+int
+yylex (yy::parser::semantic_type *, yy::location *)
+{
+  static char const *input = "aa";
+  return *input++;
+}
+
+void
+yy::parser::error (const location_type &, const std::string &m)
+{
+  std::cerr << m << std::endl;
+}
+
+int
+main (void)
+{
+  yy::parser parser;
+  return parser.parse ();
+}
+]])
+AT_BISON_CHECK([[-o input.cc input.yy]], [[0]], [[]], [[ignore-nolog]])
+AT_COMPILE_CXX([[input]])
+# This used to print "Discarding 'a'." again at the end.
+AT_PARSER_CHECK([[./input]], [[1]], [[]],
+[[syntax error
+Discarding 'a'.
+Reducing 'a'.
+]])
+
+AT_CLEANUP
-- 
1.5.4.3