Fix non-finite division handling

include/boost/decimal/decimal128_t.hpp

@@ -1899,7 +1899,7 @@ constexpr auto operator/(const decimal128_t lhs, const Integer rhs) noexcept
         case FP_NAN:
             return issignaling(lhs) ? nan_conversion(lhs) : lhs;
         case FP_INFINITE:
-            return lhs;
+            return sign ? -lhs : lhs;
         case FP_ZERO:
             return sign ? -zero : zero;
         default:

include/boost/decimal/decimal32_t.hpp

@@ -2043,7 +2043,7 @@ constexpr auto operator/(const decimal32_t lhs, Integer rhs) noexcept
         case FP_NAN:
             return issignaling(lhs) ? nan_conversion(lhs) : lhs;
         case FP_INFINITE:
-            return lhs;
+            return sign ? -lhs : lhs;
         case FP_ZERO:
             return sign ? -zero : zero;
         default:

include/boost/decimal/decimal64_t.hpp

@@ -1574,7 +1574,7 @@ constexpr auto d64_div_impl(const decimal64_t lhs, const decimal64_t rhs, decima
         switch (lhs_fp)
         {
             case FP_INFINITE:
-                if (lhs_fp == FP_INFINITE)
+                if (rhs_fp == FP_INFINITE)
                 {
                     q = nan;
                     r = nan;
@@ -1604,7 +1604,7 @@ constexpr auto d64_div_impl(const decimal64_t lhs, const decimal64_t rhs, decima
         switch (rhs_fp)
         {
             case FP_ZERO:
-                q = inf;
+                q = sign ? -inf : inf;
                 r = zero;
                 return;
             case FP_INFINITE:
@@ -1901,7 +1901,7 @@ constexpr auto operator/(const decimal64_t lhs, const Integer rhs) noexcept
         case FP_NAN:
             return issignaling(lhs) ? nan_conversion(lhs) : lhs;
         case FP_INFINITE:
-            return lhs;
+            return sign ? -lhs : lhs;
         case FP_ZERO:
             return sign ? -zero : zero;
         default:

include/boost/decimal/decimal_fast128_t.hpp

@@ -1350,7 +1350,7 @@ constexpr auto operator/(const decimal_fast128_t& lhs, const Integer rhs) noexce
         case FP_NAN:
             return issignaling(lhs) ? nan_conversion(lhs) : lhs;;
         case FP_INFINITE:
-            return lhs;
+            return sign ? -lhs : lhs;
         case FP_ZERO:
             return sign ? -zero : zero;
         default:

include/boost/decimal/decimal_fast32_t.hpp

@@ -1335,7 +1335,7 @@ constexpr auto operator/(const decimal_fast32_t lhs, const Integer rhs) noexcept
         case FP_NAN:
             return issignaling(lhs) ? nan_conversion(lhs) : lhs;
         case FP_INFINITE:
-            return lhs;
+            return sign ? -lhs : lhs;
         case FP_ZERO:
             return sign ? -zero : zero;
         default:

include/boost/decimal/decimal_fast64_t.hpp

@@ -1480,7 +1480,7 @@ constexpr auto operator/(const decimal_fast64_t lhs, const Integer rhs) noexcept
         case FP_NAN:
             return issignaling(lhs) ? nan_conversion(lhs) : lhs;;
         case FP_INFINITE:
-            return lhs;
+            return sign ? -lhs : lhs;
         case FP_ZERO:
             return sign ? -zero : zero;
         default:

test/Jamfile

@@ -87,6 +87,7 @@ run github_issue_1299.cpp ;
 run github_issue_1302.cpp ;
 run github_issue_1304.cpp ;
 run github_issue_1306.cpp ;
+run github_issue_1312.cpp ;
 
 run link_1.cpp link_2.cpp link_3.cpp ;
 run quick.cpp ;

test/github_issue_1312.cpp

@@ -0,0 +1,37 @@
+// 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/1312
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <limits>
+
+using namespace boost::decimal;
+
+template <typename T>
+void test()
+{
+    constexpr T inf {std::numeric_limits<T>::infinity()};
+
+    BOOST_TEST(isnan(inf / -inf));
+    BOOST_TEST_EQ(inf / -1000, -inf);
+    BOOST_TEST_EQ(inf / -T{1000}, -inf);
+    BOOST_TEST_EQ(inf / 0, inf);
+    BOOST_TEST_EQ(inf / T{0}, inf);
+    BOOST_TEST_EQ(0 / inf, T{0});
+}
+
+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();
+}

test/random_decimal64_math.cpp

@@ -611,6 +611,8 @@ int main()
 
     spot_mixed_division(4930, -24419);
 
+    BOOST_TEST_EQ(decimal64_t{"Inf"} / decimal64_t{-1000}, -std::numeric_limits<decimal64_t>::infinity());
+
     return boost::report_errors();
 }