Fix issue with temporaries when compiling constexpr expression templates with clang

Author: marcoffee

include/boost/multiprecision/detail/number_base.hpp

@@ -457,7 +457,7 @@ struct expression_storage_base<T, true>
 };
 
 template <class T>
-struct expression_storage : public expression_storage_base<T, boost::multiprecision::detail::is_arithmetic<T>::value>
+struct expression_storage : public expression_storage_base<T, boost::multiprecision::detail::is_arithmetic<T>::value || std::is_empty<T>::value>
 {};
 
 template <class T>

test/Jamfile.v2

@@ -1062,6 +1062,7 @@ test-suite misc :
       [ run constexpr_test_cpp_int_5.cpp : : : [ requires cxx14_constexpr cxx17_if_constexpr ] [ check-target-builds ../config//has_is_constant_evaluated : : <build>no ] ]
       [ run constexpr_test_cpp_int_6.cpp : : : [ requires cxx14_constexpr cxx17_if_constexpr ] [ check-target-builds ../config//has_is_constant_evaluated : "<toolset>msvc:<cxxflags>-constexpr:steps10000000" <toolset>clang:<cxxflags>-fconstexpr-steps=268435456 : <build>no ] [ check-target-builds ../config//has_constexpr_limits : <cxxflags>-fconstexpr-ops-limit=268435456 ] ]
       [ run constexpr_test_cpp_int_7.cpp : : : [ requires cxx14_constexpr cxx17_if_constexpr ] [ check-target-builds ../config//has_is_constant_evaluated : "<toolset>msvc:<cxxflags>-constexpr:steps10000000" <toolset>clang:<cxxflags>-fconstexpr-steps=268435456 : <build>no ] ]
+      [ run constexpr_test_cpp_int_8.cpp : : : [ requires cxx14_constexpr cxx17_if_constexpr ] [ check-target-builds ../config//has_is_constant_evaluated : : <build>no ] ]
 
       [ compile test_nothrow_cpp_int.cpp ]
       [ compile test_nothrow_cpp_rational.cpp ]

test/constexpr_test_cpp_int_8.cpp

@@ -0,0 +1,38 @@
+//  (C) Copyright Marco Ribeiro 2026.
+//  Use, modification and distribution are subject to the
+//  Boost Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include "constexpr_arithmetric_test.hpp"
+#include "boost/multiprecision/cpp_int.hpp"
+#include "test.hpp"
+
+#if !defined(BOOST_MP_NO_CONSTEXPR_DETECTION) && !defined(DISABLE_TESTS)
+
+int main()
+{
+   // ensure that 
+   namespace mp = boost::multiprecision;
+
+   using int512_et_off = mp::number<mp::cpp_int_backend<
+      512, 512, mp::signed_magnitude, mp::unchecked, void
+   >, mp::et_off>;
+
+   using int512_et_on = mp::number<mp::cpp_int_backend<
+      512, 512, mp::signed_magnitude, mp::unchecked, void
+   >, mp::et_on>;
+
+   static_assert(mp::abs(int512_et_off(-12345)) == 12345);
+   static_assert(mp::conj(int512_et_off(100)) == 100);
+   static_assert(mp::proj(int512_et_off(100)) == 100);
+   static_assert(mp::powm(int512_et_off(-5), 2, 7) == 4);
+
+   static_assert(mp::abs(int512_et_on(-12345)) == 12345);
+   static_assert(mp::conj(int512_et_on(100)) == 100);
+   static_assert(mp::proj(int512_et_on(100)) == 100);
+   static_assert(mp::powm(int512_et_on(-5), 2, 7) == 4);
+}
+#else
+int main() {}
+#endif
+