static constexpr generic_error_category generic_category_instance{};
};
+// Before C++17 it was mandatory to redeclare all static constexpr
+#if defined(BOOST_NO_CXX17_INLINE_VARIABLES)
template<class T> constexpr system_error_category cat_holder<T>::system_category_instance;
template<class T> constexpr generic_error_category cat_holder<T>::generic_category_instance;
+#endif
} // namespace detail