Add test set

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 ;

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