From a6c0b0bb6ab5f667e1f8cb049ba705ed7e86f8be Mon Sep 17 00:00:00 2001
From: Xeverous <20820409+Xeverous@users.noreply.github.com>
Date: Fri, 3 Oct 2025 16:53:42 +0200
Subject: [PATCH] X3: rollback iterator when rule fails to parse

This commit adjusts work of commit 2db3fde.
It corrected error_handler which should not skip whitespaces but
introduced a regression where handlers returning fail were still
moving the iterator forward. This commit adds iterator rollback
and synchronizes rule's and guard's iterators behavior.
---
 .../home/x3/nonterminal/detail/rule.hpp       |  5 ++++-
 test/x3/rule4.cpp                             | 21 +++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/include/boost/spirit/home/x3/nonterminal/detail/rule.hpp b/include/boost/spirit/home/x3/nonterminal/detail/rule.hpp
index b44a92400..422355c95 100644
--- a/include/boost/spirit/home/x3/nonterminal/detail/rule.hpp
+++ b/include/boost/spirit/home/x3/nonterminal/detail/rule.hpp
@@ -236,13 +236,16 @@ namespace boost { namespace spirit { namespace x3 { namespace detail
         {
             for (;;)
             {
+                Iterator it = first;
+
             #if BOOST_SPIRIT_X3_THROW_EXPECTATION_FAILURE
                 try
             #endif
                 {
                     if (parse_rhs_main(
-                        rhs, first, last, context, rcontext, attr, mpl::false_()))
+                        rhs, it, last, context, rcontext, attr, mpl::false_()))
                     {
+                        first = it;
                         return true;
                     }
                 }
diff --git a/test/x3/rule4.cpp b/test/x3/rule4.cpp
index df04e5c09..4c4a983a7 100644
--- a/test/x3/rule4.cpp
+++ b/test/x3/rule4.cpp
@@ -12,6 +12,7 @@
 #include <string>
 #include <cstring>
 #include <iostream>
+#include <vector>
 #include "test.hpp"
 
 namespace x3 = boost::spirit::x3;
@@ -90,6 +91,8 @@ main()
     using boost::spirit::x3::rule;
     using boost::spirit::x3::int_;
     using boost::spirit::x3::lit;
+    using boost::spirit::x3::lexeme;
+    using boost::spirit::x3::eol;
 
     { // show that ra = rb and ra %= rb works as expected
         rule<class a, int> ra;
@@ -144,6 +147,8 @@ main()
         auto r = rule<my_rule_class, char const*>()
             = '(' > int_ > ',' > int_ > ')';
 
+        got_it = 0;
+
         BOOST_TEST(test("(123,456)", r));
         BOOST_TEST(!test("(abc,def)", r));
         BOOST_TEST(!test("(123,456]", r));
@@ -153,6 +158,22 @@ main()
         BOOST_TEST(got_it == 1);
     }
 
+    { // regression test for #833
+      // rules which return error_handler_result::fail should trigger iterator rollback
+
+        auto string_literal = rule<my_rule_class, std::string>()
+            = lexeme['"' > *~char_("\"\n\r") > '"'];
+        auto r = rule<class r_id, std::vector<std::string>>()
+            = *string_literal > eol;
+
+        got_it = 0;
+
+        BOOST_TEST(test("\"abc\"\n", r));
+        BOOST_TEST_THROWS(test("\"abc\n", r), x3::expectation_failure<char const*>);
+
+        BOOST_TEST(got_it == 1);
+    }
+
     { // on_success gets pre-skipped iterator
         auto r = rule<on_success_gets_preskipped_iterator, char const*>()
             = lit("b");