From 9bd3fac954ca7a54af6d3757ebfd82038644e0b3 Mon Sep 17 00:00:00 2001
From: Matt Borland <matt@mattborland.com>
Date: Tue, 10 Feb 2026 10:57:20 -0500
Subject: [PATCH 1/3] Add test set

---
 test/Jamfile                     |  1 +
 test/github_issue_int128_320.cpp | 55 ++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)
 create mode 100644 test/github_issue_int128_320.cpp

diff --git a/test/Jamfile b/test/Jamfile
index 57f19c57..e542ed77 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -74,3 +74,4 @@ run github_issue_266.cpp ;
 run github_issue_267.cpp ;
 run github_issue_280.cpp ;
 run github_issue_282.cpp ;
+run github_issue_int128_320.cpp ;
diff --git a/test/github_issue_int128_320.cpp b/test/github_issue_int128_320.cpp
new file mode 100644
index 00000000..dc54a654
--- /dev/null
+++ b/test/github_issue_int128_320.cpp
@@ -0,0 +1,55 @@
+// Copyright 2026 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+//
+// See: https://github.com/cppalliance/int128/issues/320
+
+#include <boost/charconv.hpp>
+
+#ifdef BOOST_CHARCONV_HAS_INT128
+
+#include <boost/core/lightweight_test.hpp>
+#include <string>
+#include <cstdlib>
+
+void toChars(char* ptr, char* ptrEnd, boost::int128_type i128)
+{
+    auto r = boost::charconv::to_chars(ptr, ptrEnd, i128);
+    *r.ptr = '\0';
+}
+
+void toChars(char* ptr, char* ptrEnd, boost::uint128_type u128)
+{
+    auto r = boost::charconv::to_chars(ptr, ptrEnd, u128);
+    *r.ptr = '\0';
+}
+
+int main()
+{
+    boost::int128_type i128 = -123;
+    boost::uint128_type u128 = 123;
+
+    std::int64_t ref_value = -123;
+
+    for (int i = 1; i < 5; ++i)
+    {
+        i128 *= i;
+        u128 *= static_cast<unsigned>(i);
+        ref_value *= i;
+
+        char buf[64] = {};
+        toChars(buf, buf + 64, i128);
+        BOOST_TEST_CSTR_EQ(buf, std::to_string(ref_value).c_str());
+
+        toChars(buf, buf + 64, u128);
+        BOOST_TEST_CSTR_EQ(buf, std::to_string(std::abs(ref_value)).c_str());
+    };
+
+    return boost::report_errors();
+}
+
+#else
+
+int main() { return 0; }
+
+#endif

From c1e9caad96d21f462e123d8352370313f618c189 Mon Sep 17 00:00:00 2001
From: Matt Borland <matt@mattborland.com>
Date: Tue, 10 Feb 2026 10:57:53 -0500
Subject: [PATCH 2/3] Pass unsigned value instead of signed value

---
 include/boost/charconv/detail/to_chars_integer_impl.hpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/boost/charconv/detail/to_chars_integer_impl.hpp b/include/boost/charconv/detail/to_chars_integer_impl.hpp
index fe42c456..5b256c26 100644
--- a/include/boost/charconv/detail/to_chars_integer_impl.hpp
+++ b/include/boost/charconv/detail/to_chars_integer_impl.hpp
@@ -270,7 +270,7 @@ BOOST_CHARCONV_CONSTEXPR to_chars_result to_chars_128integer_impl(char* first, c
     // If the value fits into 64 bits use the other method of processing
     if (converted_value < (std::numeric_limits<std::uint64_t>::max)())
     {
-        return to_chars_integer_impl(first, last, static_cast<std::uint64_t>(value));
+        return to_chars_integer_impl(first, last, static_cast<std::uint64_t>(converted_value));
     }
 
     constexpr std::uint32_t ten_9 = UINT32_C(1000000000);

From f3f60d24f73f9744366bdf3c025cdb8a5da2e361 Mon Sep 17 00:00:00 2001
From: Matt Borland <matt@mattborland.com>
Date: Tue, 10 Feb 2026 11:16:28 -0500
Subject: [PATCH 3/3] Ignore GCC-12 stringop overflow warning in test set

---
 test/github_issue_int128_320.cpp | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/test/github_issue_int128_320.cpp b/test/github_issue_int128_320.cpp
index dc54a654..4d4842da 100644
--- a/test/github_issue_int128_320.cpp
+++ b/test/github_issue_int128_320.cpp
@@ -8,6 +8,11 @@
 
 #ifdef BOOST_CHARCONV_HAS_INT128
 
+#if defined(__GNUC__) && __GNUC__ == 12
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wstringop-overflow"
+#endif
+
 #include <boost/core/lightweight_test.hpp>
 #include <string>
 #include <cstdlib>