Round a short decimal128_t product near the smallest exponent

include/boost/decimal/detail/mul_impl.hpp

@@ -500,7 +500,7 @@ BOOST_DECIMAL_CUDA_CONSTEXPR auto d128_mul_impl(const T1& lhs_sig_in, const U1 l
         auto res_sig {detail::umul256(lhs_sig_in, rhs_sig_in)};
         auto res_exp_mut {static_cast<typename ReturnType::biased_exponent_type>(lhs_exp_in + rhs_exp_in)};
         const auto sig_dig {detail::num_digits(res_sig)};
-        const auto digit_delta {sig_dig - std::numeric_limits<sig_type>::digits10};
+        const auto digit_delta {sig_dig - detail::precision_v<ReturnType>};
         if (BOOST_DECIMAL_LIKELY(digit_delta > 0))
         {
             auto biased_exp {res_exp_mut + detail::bias_v<ReturnType>};

test/Jamfile

@@ -143,6 +143,7 @@ run github_issue_1459_toward_zero.cpp
 run github_issue_1467.cpp ;
 run github_issue_1473.cpp ;
 run github_issue_1476.cpp ;
+run github_issue_1482.cpp ;
 
 run link_1.cpp link_2.cpp link_3.cpp ;
 run quick.cpp ;

test/github_issue_1482.cpp

@@ -0,0 +1,54 @@
+// Copyright 2026 Shen-Ta Hsieh
+// 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>
+
+using namespace boost::decimal;
+using namespace boost::decimal::literals;
+
+// The exponents sum to below -6142, and the product of the short operands has 36 to 38
+// digits. The multiply must round it to 34 digits before it packs the result.
+int main()
+{
+    BOOST_TEST_EQ(34e-6168_DL * 6304777571276319608815211782449451e-6_DL, 2143624374233948666997172006032813e-6172_DL);
+    BOOST_TEST_EQ(99e-6150_DL * 1234567890123456789012345678901234e-20_DL, 1222222211222222221122222222112222e-6168_DL);
+    BOOST_TEST_EQ(9999e-6160_DL * 9999999999999999999999999999999999e-10_DL, 9998999999999999999999999999999999e-6166_DL);
+    // A tie goes to the even neighbour
+    BOOST_TEST_EQ(5e-6160_DL * 2000000000000000000000000000000001e-10_DL, 1000000000000000000000000000000000e-6169_DL);
+
+    // A 38-digit product far below the smallest subnormal rounds to zero
+    BOOST_TEST_EQ(9999999999999999999e-3112_DL * 9999999999999999999e-3113_DL, 0_DL);
+
+    // fesetround has an effect only with the detection of constant evaluation
+    #ifndef BOOST_DECIMAL_NO_CONSTEVAL_DETECTION
+    // A positive product goes up, a negative product goes toward zero
+    fesetround(rounding_mode::fe_dec_upward);
+    BOOST_TEST_EQ(34e-6168_DL * 6304777571276319608815211782449451e-6_DL, 2143624374233948666997172006032814e-6172_DL);
+    BOOST_TEST_EQ(9999e-6160_DL * 9999999999999999999999999999999999e-10_DL, 9999000000000000000000000000000000e-6166_DL);
+    BOOST_TEST_EQ(-5e-6160_DL * 2000000000000000000000000000000001e-10_DL, -1000000000000000000000000000000000e-6169_DL);
+
+    // A negative product goes down, a positive product goes toward zero
+    fesetround(rounding_mode::fe_dec_downward);
+    BOOST_TEST_EQ(-34e-6168_DL * 6304777571276319608815211782449451e-6_DL, -2143624374233948666997172006032814e-6172_DL);
+    BOOST_TEST_EQ(-9999e-6160_DL * 9999999999999999999999999999999999e-10_DL, -9999000000000000000000000000000000e-6166_DL);
+    BOOST_TEST_EQ(99e-6150_DL * 1234567890123456789012345678901234e-20_DL, 1222222211222222221122222222112221e-6168_DL);
+
+    // A tie goes away from zero
+    fesetround(rounding_mode::fe_dec_to_nearest_from_zero);
+    BOOST_TEST_EQ(5e-6160_DL * 2000000000000000000000000000000001e-10_DL, 1000000000000000000000000000000001e-6169_DL);
+    BOOST_TEST_EQ(-5e-6160_DL * 2000000000000000000000000000000001e-10_DL, -1000000000000000000000000000000001e-6169_DL);
+    BOOST_TEST_EQ(99e-6150_DL * 1234567890123456789012345678901234e-20_DL, 1222222211222222221122222222112222e-6168_DL);
+
+    // All products go toward zero
+    fesetround(rounding_mode::fe_dec_toward_zero);
+    BOOST_TEST_EQ(99e-6150_DL * 1234567890123456789012345678901234e-20_DL, 1222222211222222221122222222112221e-6168_DL);
+    BOOST_TEST_EQ(-9999e-6160_DL * 9999999999999999999999999999999999e-10_DL, -9998999999999999999999999999999999e-6166_DL);
+    BOOST_TEST_EQ(5e-6160_DL * 2000000000000000000000000000000001e-10_DL, 1000000000000000000000000000000000e-6169_DL);
+
+    fesetround(rounding_mode::fe_dec_to_nearest);
+    #endif
+
+    return boost::report_errors();
+}