Fix case of finite x - inf not returning -inf

include/boost/decimal/decimal128_t.hpp

@@ -1709,6 +1709,10 @@ constexpr auto operator-(const decimal128_t& lhs, const decimal128_t& rhs) noexc
         {
             return from_bits(detail::d128_nan_mask);
         }
+        if (isinf(rhs) && !isnan(lhs))
+        {
+            return -rhs;
+        }
 
         return detail::check_non_finite(lhs, rhs);
     }
@@ -1755,6 +1759,11 @@ constexpr auto operator-(const Integer lhs, const decimal128_t rhs) noexcept
     #ifndef BOOST_DECIMAL_FAST_MATH
     if (not_finite(rhs))
     {
+        if (isinf(rhs))
+        {
+            return -rhs;
+        }
+
         return detail::check_non_finite(rhs);
     }
     #endif

include/boost/decimal/decimal32_t.hpp

@@ -1088,6 +1088,10 @@ constexpr auto operator-(const decimal32_t lhs, const decimal32_t rhs) noexcept
         {
             return from_bits(detail::d32_nan_mask);
         }
+        if (isinf(rhs) && !isnan(lhs))
+        {
+            return -rhs;
+        }
 
         return detail::check_non_finite(lhs, rhs);
     }
@@ -1144,6 +1148,11 @@ constexpr auto operator-(const Integer lhs, const decimal32_t rhs) noexcept
     #ifndef BOOST_DECIMAL_FAST_MATH
     if (!isfinite(rhs))
     {
+        if (isinf(rhs))
+        {
+            return -rhs;
+        }
+
         return detail::check_non_finite(rhs);
     }
     #endif

include/boost/decimal/decimal64_t.hpp

@@ -1693,7 +1693,11 @@ constexpr auto operator-(const decimal64_t lhs, const decimal64_t rhs) noexcept
         {
             return from_bits(detail::d64_nan_mask);
         }
-
+        if (isinf(rhs) && !isnan(lhs))
+        {
+            return -rhs;
+        }
+
         return detail::check_non_finite(lhs, rhs);
     }
     #endif
@@ -1746,6 +1750,11 @@ constexpr auto operator-(const Integer lhs, const decimal64_t rhs) noexcept
     #ifndef BOOST_DECIMAL_FAST_MATH
     if (not_finite(rhs))
     {
+        if (isinf(rhs))
+        {
+            return -rhs;
+        }
+
         return detail::check_non_finite(rhs);
     }
     #endif

include/boost/decimal/decimal_fast128_t.hpp

@@ -1076,6 +1076,10 @@ constexpr auto operator-(const decimal_fast128_t& lhs, const decimal_fast128_t&
         {
             return direct_init_d128(detail::d128_fast_qnan, 0, false);
         }
+        if (isinf(rhs) && !isnan(lhs))
+        {
+            return -rhs;
+        }
 
         return detail::check_non_finite(lhs, rhs);
     }
@@ -1113,6 +1117,11 @@ constexpr auto operator-(const Integer lhs, const decimal_fast128_t& rhs) noexce
     #ifndef BOOST_DECIMAL_FAST_MATH
     if (not_finite(rhs))
     {
+        if (isinf(rhs))
+        {
+            return -rhs;
+        }
+
         return detail::check_non_finite(rhs);
     }
     #endif

include/boost/decimal/decimal_fast32_t.hpp

@@ -1052,6 +1052,10 @@ constexpr auto operator-(const decimal_fast32_t lhs, decimal_fast32_t rhs) noexc
         {
             return direct_init(detail::d32_fast_qnan, UINT8_C((0)));
         }
+        if (isinf(rhs) && !isnan(lhs))
+        {
+            return -rhs;
+        }
 
         return detail::check_non_finite(lhs, rhs);
     }
@@ -1098,6 +1102,11 @@ constexpr auto operator-(const Integer lhs, const decimal_fast32_t rhs) noexcept
     #ifndef BOOST_DECIMAL_FAST_MATH
     if (!isfinite(rhs))
     {
+        if (isinf(rhs))
+        {
+            return -rhs;
+        }
+
         return detail::check_non_finite(rhs);
     }
     #endif

include/boost/decimal/decimal_fast64_t.hpp

@@ -1177,6 +1177,10 @@ constexpr auto operator-(const decimal_fast64_t lhs, decimal_fast64_t rhs) noexc
         {
             return direct_init_d64(detail::d64_fast_qnan, 0, false);
         }
+        if (isinf(rhs) && !isnan(lhs))
+        {
+            return -rhs;
+        }
 
         return detail::check_non_finite(lhs, rhs);
     }
@@ -1223,6 +1227,11 @@ constexpr auto operator-(const Integer lhs, const decimal_fast64_t rhs) noexcept
     #ifndef BOOST_DECIMAL_FAST_MATH
     if (not_finite(rhs))
     {
+        if (isinf(rhs))
+        {
+            return -rhs;
+        }
+
         return detail::check_non_finite(rhs);
     }
     #endif

test/Jamfile

@@ -85,6 +85,7 @@ run github_issue_1260.cpp ;
 run github_issue_1294.cpp ;
 run github_issue_1299.cpp ;
 run github_issue_1302.cpp ;
+run github_issue_1304.cpp ;
 
 run link_1.cpp link_2.cpp link_3.cpp ;
 run quick.cpp ;

test/github_issue_1304.cpp

@@ -0,0 +1,42 @@
+// 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/decimal/issues/1304
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <limits>
+
+using namespace boost::decimal;
+
+template <typename T>
+void test()
+{
+    BOOST_TEST_EQ(std::numeric_limits<T>::infinity() - T{1000}, std::numeric_limits<T>::infinity());
+    BOOST_TEST_EQ(std::numeric_limits<T>::infinity() - 1000, std::numeric_limits<T>::infinity());
+    BOOST_TEST_EQ(-std::numeric_limits<T>::infinity() - T{1000}, -std::numeric_limits<T>::infinity());
+    BOOST_TEST_EQ(-std::numeric_limits<T>::infinity() - 1000, -std::numeric_limits<T>::infinity());
+    BOOST_TEST_EQ(-T{1000} - std::numeric_limits<T>::infinity(), -std::numeric_limits<T>::infinity());
+    BOOST_TEST_EQ(-1000 - std::numeric_limits<T>::infinity(), -std::numeric_limits<T>::infinity());
+    BOOST_TEST_EQ(T{1000} - std::numeric_limits<T>::infinity(), -std::numeric_limits<T>::infinity());
+    BOOST_TEST_EQ(1000 - std::numeric_limits<T>::infinity(), -std::numeric_limits<T>::infinity());
+
+    BOOST_TEST(isnan(std::numeric_limits<T>::quiet_NaN() - std::numeric_limits<T>::infinity()));
+    BOOST_TEST(isnan(std::numeric_limits<T>::infinity() - std::numeric_limits<T>::quiet_NaN()));
+    BOOST_TEST(isnan(std::numeric_limits<T>::signaling_NaN() - std::numeric_limits<T>::infinity()));
+    BOOST_TEST(isnan(std::numeric_limits<T>::infinity() - std::numeric_limits<T>::signaling_NaN()));
+}
+
+int main()
+{
+    test<decimal32_t>();
+    test<decimal64_t>();
+    test<decimal128_t>();
+
+    test<decimal_fast32_t>();
+    test<decimal_fast64_t>();
+    test<decimal_fast128_t>();
+
+    return boost::report_errors();
+}