diff --git a/sprout/generator/detail/generated_value.hpp b/sprout/generator/detail/generated_value.hpp
new file mode 100644
index 00000000..bc10f3bc
--- /dev/null
+++ b/sprout/generator/detail/generated_value.hpp
@@ -0,0 +1,226 @@
+/*=============================================================================
+  Copyright (c) 2011-2016 Bolero MURAKAMI
+  https://github.com/bolero-MURAKAMI/Sprout
+
+  Distributed under 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)
+=============================================================================*/
+#ifndef SPROUT_GENERATOR_DETAIL_GENERATED_VALUE_HPP
+#define SPROUT_GENERATOR_DETAIL_GENERATED_VALUE_HPP
+
+#include <utility>
+#include <type_traits>
+#include <sprout/config.hpp>
+#include <sprout/type_traits/integral_constant.hpp>
+#include <sprout/type_traits/identity.hpp>
+#include <sprout/iterator/const_reference_cast.hpp>
+#include <sprout/utility/as_const.hpp>
+#include <sprout/utility/move.hpp>
+#include <sprout/tuple/tuple.hpp>
+#include <sprout/adl/not_found.hpp>
+
+namespace sprout_adl {
+	sprout::not_found_via_adl get_generated_value(...);
+}	// namespace sprout_adl
+
+namespace sprout {
+	namespace detail {
+		template<typename T>
+		struct has_mem_generated_value_test {
+		public:
+			template<
+				typename U = T,
+				typename = typename sprout::identity<decltype(std::declval<U>().generated_value())>::type
+			>
+			static sprout::true_type test(int);
+			static sprout::false_type test(...);
+		};
+#if defined(_MSC_VER) && (_MSC_VER > 1900)
+		template<typename T, typename Base_ = typename sprout::identity<decltype(sprout::detail::has_mem_generated_value_test<T>::test(0))>::type>
+		struct has_mem_generated_value
+			: public Base_
+		{};
+#else
+		template<typename T>
+		struct has_mem_generated_value
+			: public sprout::identity<decltype(sprout::detail::has_mem_generated_value_test<T>::test(0))>::type
+		{};
+#endif
+
+		template<typename T>
+		struct is_substitutable_const_generated_value
+			: public sprout::bool_constant<
+				sprout::detail::has_mem_generated_value<T const>::value
+			>
+		{};
+
+		template<typename Gen, typename = void>
+		struct get_generated_value_impl;
+
+		template<typename Gen>
+		struct get_generated_value_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_lvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& sprout::detail::is_substitutable_const_generated_value<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen&>().generated_value())
+			get_generated_value(Gen& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen&>().generated_value())
+			{
+				typedef decltype(std::declval<Gen&>().generated_value()) type;
+				return sprout::const_reference_cast<type>(sprout::as_const(gen).generated_value());
+			}
+		};
+		template<typename Gen>
+		struct get_generated_value_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_lvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::is_substitutable_const_generated_value<Gen>::value
+					&& sprout::detail::has_mem_generated_value<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen&>().generated_value())
+			get_generated_value(Gen& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen&>().generated_value())
+			{
+				return gen.generated_value();
+			}
+		};
+		template<typename Gen>
+		struct get_generated_value_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_lvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::is_substitutable_const_generated_value<Gen>::value
+					&& !sprout::detail::has_mem_generated_value<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(sprout::tuples::get<0>(std::declval<Gen&>()))
+			get_generated_value(Gen& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(sprout::tuples::get<0>(std::declval<Gen&>()))
+			{
+				return sprout::tuples::get<0>(gen);
+			}
+		};
+
+		template<typename Gen>
+		struct get_generated_value_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_rvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& sprout::detail::is_substitutable_const_generated_value<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen&&>().generated_value())
+			get_generated_value(Gen&& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen&&>().generated_value())
+			{
+				typedef decltype(std::declval<Gen&&>().generated_value()) type;
+				return sprout::const_reference_cast<type>(sprout::as_const(sprout::move(gen)).generated_value());
+			}
+		};
+		template<typename Gen>
+		struct get_generated_value_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_rvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::is_substitutable_const_generated_value<Gen>::value
+					&& sprout::detail::has_mem_generated_value<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen&&>().generated_value())
+			get_generated_value(Gen&& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen&&>().generated_value())
+			{
+				return sprout::move(gen).generated_value();
+			}
+		};
+		template<typename Gen>
+		struct get_generated_value_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_rvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::is_substitutable_const_generated_value<Gen>::value
+					&& !sprout::detail::has_mem_generated_value<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(sprout::tuples::get<0>(std::declval<Gen&&>()))
+			get_generated_value(Gen&& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(sprout::tuples::get<0>(std::declval<Gen&&>()))
+			{
+				return sprout::tuples::get<0>(sprout::move(gen));
+			}
+		};
+
+		template<typename Gen>
+		struct get_generated_value_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_reference<Gen>::value && std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& sprout::detail::has_mem_generated_value<Gen const>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen const&>().generated_value())
+			get_generated_value(Gen const& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen const&>().generated_value())
+			{
+				return gen.generated_value();
+			}
+		};
+		template<typename Gen>
+		struct get_generated_value_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_reference<Gen>::value && std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::has_mem_generated_value<Gen const>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(sprout::tuples::get<0>(std::declval<Gen const&>()))
+			get_generated_value(Gen const& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(sprout::tuples::get<0>(std::declval<Gen const&>()))
+			{
+				return sprout::tuples::get<0>(gen);
+			}
+		};
+	}	// namespace detail
+}	// namespace sprout
+
+namespace sprout_generator_detail {
+	template<typename Gen>
+	inline SPROUT_CONSTEXPR decltype(sprout::detail::get_generated_value_impl<Gen&>::get_generated_value(std::declval<Gen&>()))
+	get_generated_value(Gen& gen)
+	SPROUT_NOEXCEPT_IF_EXPR(sprout::detail::get_generated_value_impl<Gen&>::get_generated_value(std::declval<Gen&>()))
+	{
+		return sprout::detail::get_generated_value_impl<Gen&>::get_generated_value(gen);
+	}
+	template<typename Gen>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		!std::is_const<Gen>::value && !std::is_volatile<Gen>::value && !std::is_reference<Gen>::value,
+		decltype(sprout::detail::get_generated_value_impl<Gen&&>::get_generated_value(std::declval<Gen&&>()))
+	>::type
+	get_generated_value(Gen&& gen)
+	SPROUT_NOEXCEPT_IF_EXPR(sprout::detail::get_generated_value_impl<Gen&&>::get_generated_value(std::declval<Gen&&>()))
+	{
+		return sprout::detail::get_generated_value_impl<Gen&&>::get_generated_value(sprout::move(gen));
+	}
+	template<typename Gen>
+	inline SPROUT_CONSTEXPR decltype(sprout::detail::get_generated_value_impl<Gen const&>::get_generated_value(std::declval<Gen const&>()))
+	get_generated_value(Gen const& gen)
+	SPROUT_NOEXCEPT_IF_EXPR(sprout::detail::get_generated_value_impl<Gen const&>::get_generated_value(std::declval<Gen const&>()))
+	{
+		return sprout::detail::get_generated_value_impl<Gen const&>::get_generated_value(gen);
+	}
+}	// namespace sprout_generator_detail
+
+#endif	// #ifndef SPROUT_GENERATOR_DETAIL_GENERATED_VALUE_HPP
diff --git a/sprout/generator/detail/next_generator.hpp b/sprout/generator/detail/next_generator.hpp
new file mode 100644
index 00000000..d1c603be
--- /dev/null
+++ b/sprout/generator/detail/next_generator.hpp
@@ -0,0 +1,226 @@
+/*=============================================================================
+  Copyright (c) 2011-2016 Bolero MURAKAMI
+  https://github.com/bolero-MURAKAMI/Sprout
+
+  Distributed under 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)
+=============================================================================*/
+#ifndef SPROUT_GENERATOR_DETAIL_NEXT_GENERATOR_HPP
+#define SPROUT_GENERATOR_DETAIL_NEXT_GENERATOR_HPP
+
+#include <utility>
+#include <type_traits>
+#include <sprout/config.hpp>
+#include <sprout/type_traits/integral_constant.hpp>
+#include <sprout/type_traits/identity.hpp>
+#include <sprout/iterator/const_reference_cast.hpp>
+#include <sprout/utility/as_const.hpp>
+#include <sprout/utility/move.hpp>
+#include <sprout/tuple/tuple.hpp>
+#include <sprout/adl/not_found.hpp>
+
+namespace sprout_adl {
+	sprout::not_found_via_adl get_next_generator(...);
+}	// namespace sprout_adl
+
+namespace sprout {
+	namespace detail {
+		template<typename T>
+		struct has_mem_next_generator_test {
+		public:
+			template<
+				typename U = T,
+				typename = typename sprout::identity<decltype(std::declval<U>().next_generator())>::type
+			>
+			static sprout::true_type test(int);
+			static sprout::false_type test(...);
+		};
+#if defined(_MSC_VER) && (_MSC_VER > 1900)
+		template<typename T, typename Base_ = typename sprout::identity<decltype(sprout::detail::has_mem_next_generator_test<T>::test(0))>::type>
+		struct has_mem_next_generator
+			: public Base_
+		{};
+#else
+		template<typename T>
+		struct has_mem_next_generator
+			: public sprout::identity<decltype(sprout::detail::has_mem_next_generator_test<T>::test(0))>::type
+		{};
+#endif
+
+		template<typename T>
+		struct is_substitutable_const_next_generator
+			: public sprout::bool_constant<
+				sprout::detail::has_mem_next_generator<T const>::value
+			>
+		{};
+
+		template<typename Gen, typename = void>
+		struct get_next_generator_impl;
+
+		template<typename Gen>
+		struct get_next_generator_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_lvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& sprout::detail::is_substitutable_const_next_generator<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen&>().next_generator())
+			get_next_generator(Gen& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen&>().next_generator())
+			{
+				typedef decltype(std::declval<Gen&>().next_generator()) type;
+				return sprout::const_reference_cast<type>(sprout::as_const(gen).next_generator());
+			}
+		};
+		template<typename Gen>
+		struct get_next_generator_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_lvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::is_substitutable_const_next_generator<Gen>::value
+					&& sprout::detail::has_mem_next_generator<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen&>().next_generator())
+			get_next_generator(Gen& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen&>().next_generator())
+			{
+				return gen.next_generator();
+			}
+		};
+		template<typename Gen>
+		struct get_next_generator_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_lvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::is_substitutable_const_next_generator<Gen>::value
+					&& !sprout::detail::has_mem_next_generator<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(sprout::tuples::get<1>(std::declval<Gen&>()))
+			get_next_generator(Gen& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(sprout::tuples::get<1>(std::declval<Gen&>()))
+			{
+				return sprout::tuples::get<1>(gen);
+			}
+		};
+
+		template<typename Gen>
+		struct get_next_generator_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_rvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& sprout::detail::is_substitutable_const_next_generator<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen&&>().next_generator())
+			get_next_generator(Gen&& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen&&>().next_generator())
+			{
+				typedef decltype(std::declval<Gen&&>().next_generator()) type;
+				return sprout::const_reference_cast<type>(sprout::as_const(sprout::move(gen)).next_generator());
+			}
+		};
+		template<typename Gen>
+		struct get_next_generator_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_rvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::is_substitutable_const_next_generator<Gen>::value
+					&& sprout::detail::has_mem_next_generator<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen&&>().next_generator())
+			get_next_generator(Gen&& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen&&>().next_generator())
+			{
+				return sprout::move(gen).next_generator();
+			}
+		};
+		template<typename Gen>
+		struct get_next_generator_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_rvalue_reference<Gen>::value && !std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::is_substitutable_const_next_generator<Gen>::value
+					&& !sprout::detail::has_mem_next_generator<Gen>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(sprout::tuples::get<1>(std::declval<Gen&&>()))
+			get_next_generator(Gen&& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(sprout::tuples::get<1>(std::declval<Gen&&>()))
+			{
+				return sprout::tuples::get<1>(sprout::move(gen));
+			}
+		};
+
+		template<typename Gen>
+		struct get_next_generator_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_reference<Gen>::value && std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& sprout::detail::has_mem_next_generator<Gen const>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(std::declval<Gen const&>().next_generator())
+			get_next_generator(Gen const& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen const&>().next_generator())
+			{
+				return gen.next_generator();
+			}
+		};
+		template<typename Gen>
+		struct get_next_generator_impl<
+			Gen,
+			typename std::enable_if<
+				std::is_reference<Gen>::value && std::is_const<typename std::remove_reference<Gen>::type>::value
+					&& !sprout::detail::has_mem_next_generator<Gen const>::value
+			>::type
+		> {
+		public:
+			static SPROUT_CONSTEXPR decltype(sprout::tuples::get<1>(std::declval<Gen const&>()))
+			get_next_generator(Gen const& gen)
+			SPROUT_NOEXCEPT_IF_EXPR(sprout::tuples::get<1>(std::declval<Gen const&>()))
+			{
+				return sprout::tuples::get<1>(gen);
+			}
+		};
+	}	// namespace detail
+}	// namespace sprout
+
+namespace sprout_generator_detail {
+	template<typename Gen>
+	inline SPROUT_CONSTEXPR decltype(sprout::detail::get_next_generator_impl<Gen&>::get_next_generator(std::declval<Gen&>()))
+	get_next_generator(Gen& gen)
+	SPROUT_NOEXCEPT_IF_EXPR(sprout::detail::get_next_generator_impl<Gen&>::get_next_generator(std::declval<Gen&>()))
+	{
+		return sprout::detail::get_next_generator_impl<Gen&>::get_next_generator(gen);
+	}
+	template<typename Gen>
+	inline SPROUT_CONSTEXPR typename std::enable_if<
+		!std::is_const<Gen>::value && !std::is_volatile<Gen>::value && !std::is_reference<Gen>::value,
+		decltype(sprout::detail::get_next_generator_impl<Gen&&>::get_next_generator(std::declval<Gen&&>()))
+	>::type
+	get_next_generator(Gen&& gen)
+	SPROUT_NOEXCEPT_IF_EXPR(sprout::detail::get_next_generator_impl<Gen&&>::get_next_generator(std::declval<Gen&&>()))
+	{
+		return sprout::detail::get_next_generator_impl<Gen&&>::get_next_generator(sprout::move(gen));
+	}
+	template<typename Gen>
+	inline SPROUT_CONSTEXPR decltype(sprout::detail::get_next_generator_impl<Gen const&>::get_next_generator(std::declval<Gen const&>()))
+	get_next_generator(Gen const& gen)
+	SPROUT_NOEXCEPT_IF_EXPR(sprout::detail::get_next_generator_impl<Gen const&>::get_next_generator(std::declval<Gen const&>()))
+	{
+		return sprout::detail::get_next_generator_impl<Gen const&>::get_next_generator(gen);
+	}
+}	// namespace sprout_generator_detail
+
+#endif	// #ifndef SPROUT_GENERATOR_DETAIL_NEXT_GENERATOR_HPP
diff --git a/sprout/generator/generated_value.hpp b/sprout/generator/generated_value.hpp
index ed4d9476..b0848576 100644
--- a/sprout/generator/generated_value.hpp
+++ b/sprout/generator/generated_value.hpp
@@ -20,11 +20,14 @@ namespace sprout {
 		// generated_value
 		//
 		//	effect:
-		//		sprout::generators::generator_access_traits<typename std::remove_reference<T>::type>::get_generated_value(t)
+		//		sprout::generators::generator_access_traits<T>::get_generated_value(t)
 		//		[default]
 		//			ADL callable get_generated_value(t) -> get_generated_value(t)
 		//			[default]
-		//				callable t.generated_value() -> t.generated_value()
+		//				T is not const
+		//					&& callable sprout::as_const(t).generated_value()
+		//					-> sprout::const_reference_cast<decltype(std::declval<T&>().generated_value())>(sprout::as_const(cont).generated_value())
+		//				otherwise, callable t.generated_value() -> t.generated_value()
 		//				otherwise -> sprout::tuples::get<0>(t)
 		//
 		template<typename T>
diff --git a/sprout/generator/generator_access_traits.hpp b/sprout/generator/generator_access_traits.hpp
index f8cc509b..7c34561f 100644
--- a/sprout/generator/generator_access_traits.hpp
+++ b/sprout/generator/generator_access_traits.hpp
@@ -9,166 +9,16 @@
 #define SPROUT_GENERATOR_GENERATOR_ACCESS_TRAITS_HPP
 
 #include <utility>
-#include <type_traits>
 #include <sprout/config.hpp>
-#include <sprout/utility/forward.hpp>
 #include <sprout/utility/move.hpp>
-#include <sprout/tuple/tuple.hpp>
-#include <sprout/type_traits/identity.hpp>
-#include <sprout/type_traits/integral_constant.hpp>
-#include <sprout/adl/not_found.hpp>
-
-namespace sprout_adl {
-	sprout::not_found_via_adl get_generated_value(...);
-	sprout::not_found_via_adl get_next_generator(...);
-}	// namespace sprout_adl
-
-namespace sprout {
-	namespace generators {
-		namespace detail {
-			template<typename T>
-			struct has_mem_generated_value_test {
-			public:
-				template<
-					typename U = T,
-					typename = typename sprout::identity<decltype(std::declval<U>().generated_value())>::type
-				>
-				static sprout::true_type test(int);
-				static sprout::false_type test(...);
-			};
-#if defined(_MSC_VER) && (_MSC_VER > 1900)
-			template<typename T, typename Base_ = typename sprout::identity<decltype(sprout::generators::detail::has_mem_generated_value_test<T>::test(0))>::type>
-			struct has_mem_generated_value
-				: public Base_
-			{};
-#else
-			template<typename T>
-			struct has_mem_generated_value
-				: public sprout::identity<decltype(sprout::generators::detail::has_mem_generated_value_test<T>::test(0))>::type
-			{};
-#endif
-
-			template<typename Gen, typename = void>
-			struct get_generated_value_helper;
-			template<typename Gen>
-			struct get_generated_value_helper<
-				Gen,
-				typename std::enable_if<sprout::generators::detail::has_mem_generated_value<Gen>::value>::type
-			> {
-			public:
-				static SPROUT_CONSTEXPR decltype(std::declval<Gen>().generated_value())
-				call(Gen&& t)
-				SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen>().generated_value())
-				{
-					return SPROUT_FORWARD(Gen, t).generated_value();
-				}
-			};
-			template<typename Gen>
-			struct get_generated_value_helper<
-				Gen,
-				typename std::enable_if<!sprout::generators::detail::has_mem_generated_value<Gen>::value>::type
-			> {
-			public:
-				static SPROUT_CONSTEXPR decltype(sprout::tuples::get<0>(std::declval<Gen>()))
-				call(Gen&& t)
-				SPROUT_NOEXCEPT_IF_EXPR(sprout::tuples::get<0>(std::declval<Gen>()))
-				{
-					return sprout::tuples::get<0>(SPROUT_FORWARD(Gen, t));
-				}
-			};
-			template<typename Gen>
-			inline SPROUT_CONSTEXPR decltype(sprout::generators::detail::get_generated_value_helper<Gen>::call(std::declval<Gen>()))
-			get_generated_value_impl(Gen&& t) {
-				return sprout::generators::detail::get_generated_value_helper<Gen>::call(SPROUT_FORWARD(Gen, t));
-			}
-
-			template<typename T>
-			struct has_mem_next_generator_test {
-			public:
-				template<
-					typename U = T,
-					typename = typename sprout::identity<decltype(std::declval<U>().next_generator())>::type
-				>
-				static sprout::true_type test(int);
-				static sprout::false_type test(...);
-			};
-#if defined(_MSC_VER) && (_MSC_VER > 1900)
-			template<typename T, typename Base_ = typename sprout::identity<decltype(sprout::generators::detail::has_mem_next_generator_test<T>::test(0))>::type>
-			struct has_mem_next_generator
-				: public Base_
-			{};
-#else
-			template<typename T>
-			struct has_mem_next_generator
-				: public sprout::identity<decltype(sprout::generators::detail::has_mem_next_generator_test<T>::test(0))>::type
-			{};
-#endif
-
-			template<typename Gen, typename = void>
-			struct get_next_generator_helper;
-			template<typename Gen>
-			struct get_next_generator_helper<
-				Gen,
-				typename std::enable_if<sprout::generators::detail::has_mem_next_generator<Gen>::value>::type
-			> {
-			public:
-				static SPROUT_CONSTEXPR decltype(std::declval<Gen>().next_generator())
-				call(Gen&& t)
-				SPROUT_NOEXCEPT_IF_EXPR(std::declval<Gen>().next_generator())
-				{
-					return t.next_generator();
-				}
-			};
-			template<typename Gen>
-			struct get_next_generator_helper<
-				Gen,
-				typename std::enable_if<!sprout::generators::detail::has_mem_next_generator<Gen>::value>::type
-			> {
-			public:
-				static SPROUT_CONSTEXPR decltype(sprout::tuples::get<1>(std::declval<Gen>()))
-				call(Gen&& t)
-				SPROUT_NOEXCEPT_IF_EXPR(sprout::tuples::get<1>(std::declval<Gen>()))
-				{
-					return sprout::tuples::get<1>(SPROUT_FORWARD(Gen, t));
-				}
-			};
-			template<typename Gen>
-			inline SPROUT_CONSTEXPR decltype(sprout::generators::detail::get_next_generator_helper<Gen>::call(std::declval<Gen>()))
-			get_next_generator_impl(Gen&& t) {
-				return sprout::generators::detail::get_next_generator_helper<Gen>::call(SPROUT_FORWARD(Gen, t));
-			}
-		}	// namespace detail
-	}	// namespace generators
-}	// namespace sprout
+#include <sprout/generator/traits_fwd.hpp>
+#include <sprout/generator/detail/generated_value.hpp>
+#include <sprout/generator/detail/next_generator.hpp>
 
 namespace sprout_generator_detail {
 	using sprout_adl::get_generated_value;
 	using sprout_adl::get_next_generator;
 
-	template<typename Gen>
-	inline SPROUT_CONSTEXPR decltype(sprout::generators::detail::get_generated_value_impl(std::declval<Gen&>()))
-	get_generated_value(Gen& gen)
-	SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::detail::get_generated_value_impl(std::declval<Gen&>()))
-	{
-		return sprout::generators::detail::get_generated_value_impl(gen);
-	}
-	template<typename Gen>
-	inline SPROUT_CONSTEXPR typename std::enable_if<
-		!std::is_const<Gen>::value && !std::is_volatile<Gen>::value && !std::is_reference<Gen>::value,
-		decltype(sprout::generators::detail::get_generated_value_impl(std::declval<Gen&&>()))
-	>::type
-	get_generated_value(Gen&& gen)
-	SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::detail::get_generated_value_impl(std::declval<Gen&&>()))
-	{
-		return sprout::generators::detail::get_generated_value_impl(sprout::move(gen));
-	}
-	template<typename Gen>
-	inline SPROUT_CONSTEXPR decltype(sprout::generators::detail::get_generated_value_impl(std::declval<Gen const&>()))
-	get_generated_value(Gen const& gen)
-	SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::detail::get_generated_value_impl(std::declval<Gen const&>()))
-	{
-		return sprout::generators::detail::get_generated_value_impl(gen);
-	}
 	template<typename Gen>
 	inline SPROUT_CONSTEXPR decltype(get_generated_value(std::declval<Gen>()))
 	call_get_generated_value(Gen&& gen)
@@ -176,31 +26,6 @@ namespace sprout_generator_detail {
 	{
 		return get_generated_value(SPROUT_FORWARD(Gen, gen));
 	}
-
-	template<typename Gen>
-	inline SPROUT_CONSTEXPR decltype(sprout::generators::detail::get_next_generator_impl(std::declval<Gen&>()))
-	get_next_generator(Gen& gen)
-	SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::detail::get_next_generator_impl(std::declval<Gen&>()))
-	{
-		return sprout::generators::detail::get_next_generator_impl(gen);
-	}
-	template<typename Gen>
-	inline SPROUT_CONSTEXPR typename std::enable_if<
-		!std::is_const<Gen>::value && !std::is_volatile<Gen>::value && !std::is_reference<Gen>::value,
-		decltype(sprout::generators::detail::get_next_generator_impl(std::declval<Gen&&>()))
-	>::type
-	get_next_generator(Gen&& gen)
-	SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::detail::get_next_generator_impl(std::declval<Gen&&>()))
-	{
-		return sprout::generators::detail::get_next_generator_impl(sprout::move(gen));
-	}
-	template<typename Gen>
-	inline SPROUT_CONSTEXPR decltype(sprout::generators::detail::get_next_generator_impl(std::declval<Gen const&>()))
-	get_next_generator(Gen const& gen)
-	SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::detail::get_next_generator_impl(std::declval<Gen const&>()))
-	{
-		return sprout::generators::detail::get_next_generator_impl(gen);
-	}
 	template<typename Gen>
 	inline SPROUT_CONSTEXPR decltype(get_next_generator(std::declval<Gen>()))
 	call_get_next_generator(Gen&& gen)
@@ -212,13 +37,33 @@ namespace sprout_generator_detail {
 
 namespace sprout {
 	namespace generators {
+		namespace detail {
+			template<typename Gen>
+			struct generator_access_traits_const_default {
+			public:
+				// get_generated_value
+				static SPROUT_CONSTEXPR decltype(sprout::generators::generator_access_traits<Gen>::get_generated_value(std::declval<Gen const&>()))
+				get_generated_value(Gen const& t)
+				SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::generator_access_traits<Gen>::get_generated_value(std::declval<Gen const&>()))
+				{
+					return sprout::generators::generator_access_traits<Gen>::get_generated_value(t);
+				}
+				// get_next_generator
+				static SPROUT_CONSTEXPR decltype(sprout::generators::generator_access_traits<Gen>::get_next_generator(std::declval<Gen const&>()))
+				get_next_generator(Gen const& t)
+				SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::generator_access_traits<Gen>::get_next_generator(std::declval<Gen const&>()))
+				{
+					return sprout::generators::generator_access_traits<Gen>::get_next_generator(t);
+				}
+			};
+		}	// namespace detail
+
 		//
-		// generator_access_traits
+		// generator_access_get_generated_value_default
 		//
 		template<typename Gen>
-		struct generator_access_traits {
+		struct generator_access_get_generated_value_default {
 		public:
-			// get_generated_value
 			static SPROUT_CONSTEXPR decltype(sprout_generator_detail::call_get_generated_value(std::declval<Gen&>()))
 			get_generated_value(Gen& t)
 			SPROUT_NOEXCEPT_IF_EXPR(sprout_generator_detail::call_get_generated_value(std::declval<Gen&>()))
@@ -237,7 +82,13 @@ namespace sprout {
 			{
 				return sprout_generator_detail::call_get_generated_value(t);
 			}
-			// get_next_generator
+		};
+		//
+		// generator_access_get_next_generator_default
+		//
+		template<typename Gen>
+		struct generator_access_get_next_generator_default {
+		public:
 			static SPROUT_CONSTEXPR decltype(sprout_generator_detail::call_get_next_generator(std::declval<Gen&>()))
 			get_next_generator(Gen& t)
 			SPROUT_NOEXCEPT_IF_EXPR(sprout_generator_detail::call_get_next_generator(std::declval<Gen&>()))
@@ -257,27 +108,28 @@ namespace sprout {
 				return sprout_generator_detail::call_get_next_generator(t);
 			}
 		};
-		template<typename Gen>
-		struct generator_access_traits<Gen const> {
-		public:
-			// get_generated_value
-			static SPROUT_CONSTEXPR decltype(sprout::generators::generator_access_traits<Gen>::get_generated_value(std::declval<Gen const&>()))
-			get_generated_value(Gen const& t)
-			SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::generator_access_traits<Gen>::get_generated_value(std::declval<Gen const&>()))
-			{
-				return sprout::generators::generator_access_traits<Gen>::get_generated_value(t);
-			}
-			// get_next_generator
-			static SPROUT_CONSTEXPR decltype(sprout::generators::generator_access_traits<Gen>::get_next_generator(std::declval<Gen const&>()))
-			get_next_generator(Gen const& t)
-			SPROUT_NOEXCEPT_IF_EXPR(sprout::generators::generator_access_traits<Gen>::get_next_generator(std::declval<Gen const&>()))
-			{
-				return sprout::generators::generator_access_traits<Gen>::get_next_generator(t);
-			}
-		};
-	}	// namespace generators
 
-	using sprout::generators::generator_access_traits;
+		//
+		// generator_access_traits_default
+		//
+		template<typename Gen>
+		struct generator_access_traits_default
+			: public sprout::generators::generator_access_get_generated_value_default<Gen>
+			, public sprout::generators::generator_access_get_next_generator_default<Gen>
+		{};
+
+		//
+		// generator_access_traits
+		//
+		template<typename Gen>
+		struct generator_access_traits
+			: public sprout::generators::generator_access_traits_default<Gen>
+		{};
+		template<typename Gen>
+		struct generator_access_traits<Gen const>
+			: public sprout::generators::detail::generator_access_traits_const_default<Gen>
+		{};
+	}	// namespace generators
 }	// namespace sprout
 
 #endif	// #ifndef SPROUT_GENERATOR_GENERATOR_ACCESS_TRAITS_HPP
diff --git a/sprout/generator/next_generator.hpp b/sprout/generator/next_generator.hpp
index ce11239a..f53275c8 100644
--- a/sprout/generator/next_generator.hpp
+++ b/sprout/generator/next_generator.hpp
@@ -20,11 +20,14 @@ namespace sprout {
 		// next_generator
 		//
 		//	effect:
-		//		sprout::generators::generator_access_traits<typename std::remove_reference<T>::type>::get_next_generator(t)
+		//		sprout::generators::generator_access_traits<T>::get_next_generator(t)
 		//		[default]
 		//			ADL callable get_next_generator(t) -> get_next_generator(t)
 		//			[default]
-		//				callable t.next_generator() -> t.next_generator()
+		//				T is not const
+		//					&& callable sprout::as_const(t).next_generator()
+		//					-> sprout::const_reference_cast<decltype(std::declval<T&>().next_generator())>(sprout::as_const(cont).next_generator())
+		//				otherwise, callable t.next_generator() -> t.next_generator()
 		//				otherwise -> sprout::tuples::get<0>(t)
 		//
 		template<typename T>
diff --git a/sprout/generator/traits_fwd.hpp b/sprout/generator/traits_fwd.hpp
new file mode 100644
index 00000000..676e8dab
--- /dev/null
+++ b/sprout/generator/traits_fwd.hpp
@@ -0,0 +1,25 @@
+/*=============================================================================
+  Copyright (c) 2011-2016 Bolero MURAKAMI
+  https://github.com/bolero-MURAKAMI/Sprout
+
+  Distributed under 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)
+=============================================================================*/
+#ifndef SPROUT_GENERATOR_TRAITS_FWD_HPP
+#define SPROUT_GENERATOR_TRAITS_FWD_HPP
+
+#include <sprout/config.hpp>
+
+namespace sprout {
+	namespace generators {
+		//
+		// generator_access_traits
+		//
+		template<typename Gen>
+		struct generator_access_traits;
+	}	// namespace generators
+
+	using sprout::generators::generator_access_traits;
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_GENERATOR_TRAITS_FWD_HPP