Fix narrowing in equality of mixed types

include/boost/decimal/detail/comparison.hpp

@@ -96,7 +96,7 @@ template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE DecimalType = decimal32, BOOST_DEC
 constexpr auto equal_parts_impl(T1 lhs_sig, U1 lhs_exp, bool lhs_sign,
                                 T2 rhs_sig, U2 rhs_exp, bool rhs_sign) noexcept -> std::enable_if_t<std::is_same<DecimalType, decimal32>::value || std::is_same<DecimalType, decimal64>::value || std::is_same<DecimalType, decimal128>::value, bool>
 {
-    using comp_type = typename DecimalType::significand_type;
+    using comp_type = std::conditional_t<(std::numeric_limits<T1>::digits10 > std::numeric_limits<T2>::digits10), T1, T2>;
 
     BOOST_DECIMAL_ASSERT(lhs_sig >= 0);
     BOOST_DECIMAL_ASSERT(rhs_sig >= 0);

test/Jamfile

@@ -49,6 +49,7 @@ run crash_report_1.cpp ;
 run github_issue_426.cpp ;
 run github_issue_448.cpp ;
 run-fail github_issue_519.cpp ;
+run github_issue_799.cpp ;
 run link_1.cpp link_2.cpp link_3.cpp ;
 run quick.cpp ;
 run random_decimal32_comp.cpp ;

test/github_issue_799.cpp

@@ -0,0 +1,40 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <iostream>
+
+#ifdef BOOST_DECIMAL_HAS_INT128
+
+template<typename Dec>
+void mixed_compare()
+{
+    const Dec a{2, 1};
+    const auto b = __uint128_t{std::numeric_limits<__uint128_t>::max() - std::numeric_limits<uint64_t>::max() + 20};
+    BOOST_TEST(a != b);
+}
+
+int main()
+{
+    using namespace boost::decimal;
+
+    mixed_compare<decimal32>();
+    mixed_compare<decimal32_fast>();
+    mixed_compare<decimal64>();
+    mixed_compare<decimal64_fast>();
+    mixed_compare<decimal128>();
+    mixed_compare<decimal128_fast>();
+
+    return boost::report_errors();
+}
+
+#else
+
+int main()
+{
+    return 0;
+}
+
+#endif