Question about build error in capture.cpp

I got the following build error on PPC, gcc-8.1.0, c++14.
Looks like only boost::hof::capture_forward(one)(binary_class())(two) and boost::hof::capture_forward(1)(binary_class())(2) report error.
boost::hof::capture_forward(one, two)(binary_class())() and boost::hof::capture_forward()(binary_class())(one, two) have no problem.
boost::hof::capture and boost::hof::capture_basic have no problem.
Any suggestion what is going on?

gcc.compile.c++ ../bin.v2/libs/hof/test/capture.test/gcc-8.1.0/debug/cross-compile-vxworks/static-only-on/visibility-hidden/capture.o
In file included from /boost_1_70_0/boost/hof/detail/callable_base.hpp:11,
from /boost_1_70_0/boost/hof/capture.hpp:11,
from ../libs/hof/test/capture.cpp:7:
/boost_1_70_0/boost/hof/alias.hpp: In instantiation of 'constexpr boost::hof::alias::alias(FitXs&& ...) [with FitXs = {const boost::hof::alias_inherit, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >&}; = void; T = const int&; Tag = boost::hof::detail::pack_tag, const int&>]':
type_traits:1411:20: required from 'const bool std::_Is_ntc, const int&> >, const boost::hof::alias_inherit, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >&>::value'
/boost_1_70_0/boost/hof/pack.hpp:251:15: required from 'constexpr boost::hof::detail::pack_base, Ts ...>::pack_base(Xs&& ...) [with Xs = {const boost::hof::alias_inherit, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >&}; = void; long unsigned int ...Ns = {0}; Ts = {const int&}]'
type_traits:1043:15: required by substitution of 'template static decltype (((void)((_Uty)(declval, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >&>())), std::true_type())) std::_Is_constructible, const int&>, const boost::hof::alias_inherit, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >&>::_Fn<_Uty>(int) [with _Uty = boost::hof::detail::pack_base, const int&>]'
type_traits:1050:27: required from 'struct std::_Is_constructible, const int&>, const boost::hof::alias_inherit, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >&>'
type_traits:1055:9: required from 'struct std::is_constructible, const int&>, const boost::hof::alias_inherit, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >&>'
/boost_1_70_0/boost/hof/alias.hpp:125:5: required by substitution of 'template constexpr boost::hof::alias_inherit, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >::alias_inherit(FitXs&& ...) [with FitXs = {const boost::hof::alias_inherit, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >&}; = ]'
/boost_1_70_0/boost/hof/capture.hpp:152:42: required by substitution of 'template constexpr decltype (boost::hof::detail::capture_invoke, const int&> >(static_cast(f), (boost::hof::detail::pack_base, const int&>&&)(static_cast, const int&>&>((* boost::hof::always(declval, const int&> >::fit_const_this_type>())(f)))))) boost::hof::detail::capture_pack, const int&> >::operator()(F) const [with F = binary_class]'
../libs/hof/test/capture.cpp:28:5: required from here
/boost_1_70_0/boost/hof/detail/delegate.hpp:62:46: error: invalid initialization of reference of type 'const int&' from expression of type 'const boost::hof::alias_inherit, const int&>, boost::hof::detail::pair_tag<1, boost::hof::detail::pack_base, const int&>, binary_class> >'
: var(BOOST_HOF_FORWARD(FitXs)(fit_xs)...) {}
^
/boost_1_70_0/boost/hof/detail/delegate.hpp:66:51: note: in expansion of macro 'BOOST_HOF_DELGATE_PRIMITIVE_CONSTRUCTOR'
#define BOOST_HOF_DELEGATE_CONSTRUCTOR(C, T, var) BOOST_HOF_DELGATE_PRIMITIVE_CONSTRUCTOR(constexpr, C, T, var)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/boost_1_70_0/boost/hof/alias.hpp:104:5: note: in expansion of macro 'BOOST_HOF_DELEGATE_CONSTRUCTOR'
BOOST_HOF_DELEGATE_CONSTRUCTOR(alias, T, value)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~