Fix decimal128_fast scalblnd implementation

include/boost/decimal/decimal128_fast.hpp

@@ -1370,7 +1370,7 @@ constexpr auto scalblnd128f(decimal128_fast num, long exp) noexcept -> decimal12
     }
     #endif
 
-    num.exponent_ = static_cast<decimal128_fast::exponent_type>(static_cast<long>(num.biased_exponent()) + exp);
+    num = decimal128_fast(num.significand_, num.biased_exponent() + exp, num.sign_);
 
     return num;
 }

test/Jamfile

@@ -51,6 +51,7 @@ run github_issue_448.cpp ;
 run-fail github_issue_519.cpp ;
 run github_issue_799.cpp ;
 run github_issue_802.cpp ;
+run github_issue_805.cpp ;
 run link_1.cpp link_2.cpp link_3.cpp ;
 run quick.cpp ;
 run random_decimal32_comp.cpp ;

test/github_issue_805.cpp

@@ -0,0 +1,31 @@
+// 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>
+
+using namespace boost::decimal;
+
+template <typename Dec>
+void test()
+{
+    Dec a(2, std::numeric_limits<Dec>::max_exponent10);
+    Dec b = scalbln(a, 10);
+
+    BOOST_TEST(isinf(b));
+}
+
+int main()
+{
+    test<decimal32>();
+    test<decimal64>();
+    test<decimal128>();
+    test<decimal32_fast>();
+    test<decimal64_fast>();
+    test<decimal128_fast>();
+
+    return boost::report_errors();
+}
+