From 014d339803cd937d0caf034bbd0a1b6656b26366 Mon Sep 17 00:00:00 2001
From: bolero-MURAKAMI <bolero.murakami@gmail.com>
Date: Tue, 3 Dec 2013 22:17:49 +0900
Subject: [PATCH] fix darkroom: add alpha and refraction material parameter

---
 example/darkroom/two_spheres.hpp            |  12 +-
 sprout/darkroom/colors/rgb.hpp              |  30 +++-
 sprout/darkroom/intersects/intersection.hpp |  62 ++++++--
 sprout/darkroom/materials/material.hpp      | 149 +++++++++++++++++---
 sprout/darkroom/materials/plaid.hpp         |  73 ++++++----
 sprout/darkroom/materials/uniform.hpp       |  10 +-
 sprout/utility/pack.hpp                     |   8 +-
 7 files changed, 263 insertions(+), 81 deletions(-)

diff --git a/example/darkroom/two_spheres.hpp b/example/darkroom/two_spheres.hpp
index cd2fa5e5..d194a582 100644
--- a/example/darkroom/two_spheres.hpp
+++ b/example/darkroom/two_spheres.hpp
@@ -21,26 +21,22 @@ namespace darkcult {
 			objects::aa_plane_direction::y,
 			-2.0,
 			materials::make_plaid_material_image(
-				colors::rgb_f(1.0, 0.0, 0.0),
-				colors::rgb_f(1.0, 1.0, 0.0),
-				0.0,
-				0.0
+				colors::rgb_f(1.0, 0.0, 0.0), colors::rgb_f(1.0, 1.0, 0.0),
+				0.0, 0.0
 				)
 			),
 		objects::make_sphere(
 			coords::vector3d(-1.0, 0.5, 7.5),
 			2.5,
 			materials::make_uniform_material_image(
-				colors::rgb_f(0.0, 0.0, 1.0),
-				0.2
+				colors::rgb_f(0.0, 0.0, 1.0), 0.2
 				)
 			),
 		objects::make_sphere(
 			coords::vector3d(1.0, -1.0, 4.0),
 			1.0,
 			materials::make_uniform_material_image(
-				colors::rgb_f(0.0, 1.0, 0.0),
-				0.2
+				colors::rgb_f(0.0, 1.0, 0.0), 0.2
 				)
 			)
 		);
diff --git a/sprout/darkroom/colors/rgb.hpp b/sprout/darkroom/colors/rgb.hpp
index 43df547e..43cff05e 100644
--- a/sprout/darkroom/colors/rgb.hpp
+++ b/sprout/darkroom/colors/rgb.hpp
@@ -14,11 +14,21 @@
 #include <sprout/tuple/tuple.hpp>
 #include <sprout/tuple/functions.hpp>
 #include <sprout/utility/forward.hpp>
+#include <sprout/type_traits/integral_constant.hpp>
+#include <sprout/type_traits/enabler_if.hpp>
 #include <sprout/darkroom/access/access.hpp>
 
 namespace sprout {
 	namespace darkroom {
 		namespace colors {
+			//
+			// has_alpha
+			//
+			template<typename T>
+			struct has_alpha
+				: public sprout::integral_constant<bool, (sprout::darkroom::access::size<T>::value >= 4)>
+			{};
+
 			//
 			// r
 			// g
@@ -49,7 +59,13 @@ namespace sprout {
 			{
 				return sprout::darkroom::access::get<2>(sprout::forward<T>(t));
 			}
-			template<typename T>
+			//
+			// a
+			//
+			template<
+				typename T,
+				typename sprout::enabler_if<sprout::darkroom::colors::has_alpha<T>::value>::type = sprout::enabler
+			>
 			inline SPROUT_CONSTEXPR auto
 			a(T&& t)
 			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::darkroom::access::get<3>(sprout::forward<T>(t))))
@@ -57,13 +73,22 @@ namespace sprout {
 			{
 				return sprout::darkroom::access::get<3>(sprout::forward<T>(t));
 			}
+			template<
+				typename T,
+				typename sprout::enabler_if<!sprout::darkroom::colors::has_alpha<T>::value>::type = sprout::enabler
+			>
+			inline SPROUT_CONSTEXPR typename sprout::darkroom::access::unit<T>::type
+			a(T&&)
+			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(typename sprout::darkroom::access::unit<T>::type()))
+			{
+				return typename sprout::darkroom::access::unit<T>::type();
+			}
 
 			//
 			// rgb_t
 			// rgb
 			//
 			typedef sprout::tuples::tuple<std::uint8_t, std::uint8_t, std::uint8_t> rgb_t;
-
 			inline SPROUT_CONSTEXPR sprout::darkroom::colors::rgb_t
 			rgb(std::uint8_t r = 0, std::uint8_t g = 0, std::uint8_t b = 0) {
 				return sprout::darkroom::colors::rgb_t(r, g, b);
@@ -74,7 +99,6 @@ namespace sprout {
 			// rgb_f
 			//
 			typedef sprout::tuples::tuple<double, double, double> rgb_f_t;
-
 			inline SPROUT_CONSTEXPR sprout::darkroom::colors::rgb_f_t
 			rgb_f(double r = 0, double g = 0, double b = 0) {
 				return sprout::darkroom::colors::rgb_f_t(r, g, b);
diff --git a/sprout/darkroom/intersects/intersection.hpp b/sprout/darkroom/intersects/intersection.hpp
index 235490a7..6e420303 100644
--- a/sprout/darkroom/intersects/intersection.hpp
+++ b/sprout/darkroom/intersects/intersection.hpp
@@ -11,6 +11,7 @@
 #include <sprout/config.hpp>
 #include <sprout/tuple/tuple.hpp>
 #include <sprout/utility/forward.hpp>
+#include <sprout/type_traits/enabler_if.hpp>
 #include <sprout/darkroom/access/access.hpp>
 #include <sprout/darkroom/coords/vector.hpp>
 #include <sprout/darkroom/materials/material.hpp>
@@ -18,12 +19,33 @@
 namespace sprout {
 	namespace darkroom {
 		namespace intersects {
+			//
+			// intersection
+			//
+			typedef sprout::tuples::tuple<
+				bool,
+				double,
+				sprout::darkroom::coords::vector3d_t,
+				sprout::darkroom::coords::vector3d_t,
+				sprout::darkroom::materials::material,
+				bool
+			> intersection;
+
+			//
+			// has_is_from_inside
+			//
+			template<typename T>
+			struct has_is_from_inside
+				: public sprout::integral_constant<bool, (sprout::darkroom::access::size<T>::value >= 6)>
+			{};
+
 			//
 			// does_intersect
 			// distance
 			// point_of_intersection
 			// normal
 			// material
+			// is_from_inside
 			//
 			template<typename T>
 			inline SPROUT_CONSTEXPR auto
@@ -65,26 +87,36 @@ namespace sprout {
 			{
 				return sprout::darkroom::access::get<4>(sprout::forward<T>(t));
 			}
+			template<
+				typename T,
+				typename sprout::enabler_if<sprout::darkroom::intersects::has_is_from_inside<T>::value>::type = sprout::enabler
+			>
+			inline SPROUT_CONSTEXPR auto
+			is_from_inside(T&& t)
+			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::darkroom::access::get<5>(sprout::forward<T>(t))))
+			-> decltype(sprout::darkroom::access::get<5>(sprout::forward<T>(t)))
+			{
+				return sprout::darkroom::access::get<5>(sprout::forward<T>(t));
+			}
+			template<
+				typename T,
+				typename sprout::enabler_if<!sprout::darkroom::intersects::has_is_from_inside<T>::value>::type = sprout::enabler
+			>
+			inline SPROUT_CONSTEXPR typename sprout::darkroom::access::element<5, sprout::darkroom::intersects::intersection>::type
+			is_from_inside(T&&)
+			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR((typename sprout::darkroom::access::element<5, sprout::darkroom::intersects::intersection>::type())))
+			{
+				return typename sprout::darkroom::access::element<5, sprout::darkroom::intersects::intersection>::type();
+			}
 
 			//
 			// make_intersection
 			//
-			template<typename Distance, typename Point, typename Normal, typename Material>
-			inline SPROUT_CONSTEXPR sprout::tuples::tuple<bool, Distance, Point, Normal, Material>
-			make_intersection(bool b, Distance const& dist, Point const& p, Normal const& nor, Material const& mat) {
-				return sprout::tuples::make_tuple(b, dist, p, nor, mat);
+			template<typename... Elements>
+			inline SPROUT_CONSTEXPR sprout::tuples::tuple<Elements...>
+			make_material_image(Elements const&... elems) {
+				return sprout::tuples::make_tuple(elems...);
 			}
-
-			//
-			// intersection
-			//
-			typedef sprout::tuples::tuple<
-				bool,
-				double,
-				sprout::darkroom::coords::vector3d_t,
-				sprout::darkroom::coords::vector3d_t,
-				sprout::darkroom::materials::material
-			> intersection;
 		}	// namespace intersects
 	}	// namespace darkroom
 }	// namespace sprout
diff --git a/sprout/darkroom/materials/material.hpp b/sprout/darkroom/materials/material.hpp
index 23463438..02899930 100644
--- a/sprout/darkroom/materials/material.hpp
+++ b/sprout/darkroom/materials/material.hpp
@@ -8,9 +8,12 @@
 #ifndef SPROUT_DARKROOM_MATERIALS_MATERIAL_HPP
 #define SPROUT_DARKROOM_MATERIALS_MATERIAL_HPP
 
+#include <type_traits>
 #include <sprout/config.hpp>
 #include <sprout/tuple/tuple.hpp>
+#include <sprout/tuple/indexes.hpp>
 #include <sprout/utility/forward.hpp>
+#include <sprout/type_traits/enabler_if.hpp>
 #include <sprout/darkroom/access/access.hpp>
 #include <sprout/darkroom/colors/rgb.hpp>
 #include <sprout/darkroom/materials/calculate.hpp>
@@ -19,10 +22,40 @@ namespace sprout {
 	namespace darkroom {
 		namespace materials {
 			//
-			// color
-			// reflection
+			// material
+			//
+			typedef sprout::tuples::tuple<sprout::darkroom::colors::rgb_f_t, double, double, double> material;
+
+			//
+			// has_color
+			// has_reflection
+			// has_alpha
+			// has_refraction
 			//
 			template<typename T>
+			struct has_color
+				: public sprout::integral_constant<bool, (sprout::darkroom::access::size<T>::value >= 1)>
+			{};
+			template<typename T>
+			struct has_reflection
+				: public sprout::integral_constant<bool, (sprout::darkroom::access::size<T>::value >= 2)>
+			{};
+			template<typename T>
+			struct has_alpha
+				: public sprout::integral_constant<bool, (sprout::darkroom::access::size<T>::value >= 3)>
+			{};
+			template<typename T>
+			struct has_refraction
+				: public sprout::integral_constant<bool, (sprout::darkroom::access::size<T>::value >= 4)>
+			{};
+
+			//
+			// color
+			//
+			template<
+				typename T,
+				typename sprout::enabler_if<sprout::darkroom::materials::has_color<typename std::decay<T>::type>::value>::type = sprout::enabler
+			>
 			inline SPROUT_CONSTEXPR auto
 			color(T&& t)
 			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::darkroom::access::get<0>(sprout::forward<T>(t))))
@@ -30,7 +63,23 @@ namespace sprout {
 			{
 				return sprout::darkroom::access::get<0>(sprout::forward<T>(t));
 			}
-			template<typename T>
+			template<
+				typename T,
+				typename sprout::enabler_if<!sprout::darkroom::materials::has_color<typename std::decay<T>::type>::value>::type = sprout::enabler
+			>
+			inline SPROUT_CONSTEXPR typename sprout::darkroom::access::element<0, sprout::darkroom::materials::material>::type
+			color(T&&)
+			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR((typename sprout::darkroom::access::element<0, sprout::darkroom::materials::material>::type())))
+			{
+				return typename sprout::darkroom::access::element<0, sprout::darkroom::materials::material>::type();
+			}
+			//
+			// reflection
+			//
+			template<
+				typename T,
+				typename sprout::enabler_if<sprout::darkroom::materials::has_reflection<typename std::decay<T>::type>::value>::type = sprout::enabler
+			>
 			inline SPROUT_CONSTEXPR auto
 			reflection(T&& t)
 			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::darkroom::access::get<1>(sprout::forward<T>(t))))
@@ -38,37 +87,97 @@ namespace sprout {
 			{
 				return sprout::darkroom::access::get<1>(sprout::forward<T>(t));
 			}
+			template<
+				typename T,
+				typename sprout::enabler_if<!sprout::darkroom::materials::has_reflection<typename std::decay<T>::type>::value>::type = sprout::enabler
+			>
+			inline SPROUT_CONSTEXPR typename sprout::darkroom::access::element<1, sprout::darkroom::materials::material>::type
+			reflection(T&&)
+			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR((typename sprout::darkroom::access::element<1, sprout::darkroom::materials::material>::type())))
+			{
+				return typename sprout::darkroom::access::element<1, sprout::darkroom::materials::material>::type();
+			}
+			//
+			// alpha
+			//
+			template<
+				typename T,
+				typename sprout::enabler_if<sprout::darkroom::materials::has_alpha<typename std::decay<T>::type>::value>::type = sprout::enabler
+			>
+			inline SPROUT_CONSTEXPR auto
+			alpha(T&& t)
+			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::darkroom::access::get<2>(sprout::forward<T>(t))))
+			-> decltype(sprout::darkroom::access::get<2>(sprout::forward<T>(t)))
+			{
+				return sprout::darkroom::access::get<2>(sprout::forward<T>(t));
+			}
+			template<
+				typename T,
+				typename sprout::enabler_if<!sprout::darkroom::materials::has_alpha<typename std::decay<T>::type>::value>::type = sprout::enabler
+			>
+			inline SPROUT_CONSTEXPR typename sprout::darkroom::access::element<2, sprout::darkroom::materials::material>::type
+			alpha(T&&)
+			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR((typename sprout::darkroom::access::element<2, sprout::darkroom::materials::material>::type())))
+			{
+				return typename sprout::darkroom::access::element<2, sprout::darkroom::materials::material>::type();
+			}
+			//
+			// refraction
+			//
+			template<
+				typename T,
+				typename sprout::enabler_if<sprout::darkroom::materials::has_refraction<typename std::decay<T>::type>::value>::type = sprout::enabler
+			>
+			inline SPROUT_CONSTEXPR auto
+			refraction(T&& t)
+			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR(sprout::darkroom::access::get<3>(sprout::forward<T>(t))))
+			-> decltype(sprout::darkroom::access::get<3>(sprout::forward<T>(t)))
+			{
+				return sprout::darkroom::access::get<3>(sprout::forward<T>(t));
+			}
+			template<
+				typename T,
+				typename sprout::enabler_if<!sprout::darkroom::materials::has_refraction<typename std::decay<T>::type>::value>::type = sprout::enabler
+			>
+			inline SPROUT_CONSTEXPR typename sprout::darkroom::access::element<3, sprout::darkroom::materials::material>::type
+			refraction(T&&)
+			SPROUT_NOEXCEPT_EXPR(SPROUT_NOEXCEPT_EXPR((typename sprout::darkroom::access::element<3, sprout::darkroom::materials::material>::type())))
+			{
+				return typename sprout::darkroom::access::element<1, sprout::darkroom::materials::material>::type();
+			}
 
 			//
 			// calculate_material
 			//
+			namespace detail {
+				template<typename Material, typename Unit, sprout::index_t... Indexes>
+				inline SPROUT_CONSTEXPR auto
+				calculate_material_impl(Material const& mat, Unit const& u, Unit const& v, sprout::index_tuple<Indexes...>)
+				-> decltype(sprout::tuples::make_tuple(
+					sprout::darkroom::materials::calculate(sprout::darkroom::access::get<Indexes>(mat), u, v)...
+					))
+				{
+					return sprout::tuples::make_tuple(
+						sprout::darkroom::materials::calculate(sprout::darkroom::access::get<Indexes>(mat), u, v)...
+						);
+				}
+			}	// namespace detail
 			template<typename Material, typename Unit>
 			inline SPROUT_CONSTEXPR auto
 			calculate_material(Material const& mat, Unit const& u, Unit const& v)
-			-> decltype(sprout::tuples::make_tuple(
-				sprout::darkroom::materials::calculate(sprout::darkroom::materials::color(mat), u, v),
-				sprout::darkroom::materials::calculate(sprout::darkroom::materials::reflection(mat), u, v)
-				))
+			-> decltype(sprout::darkroom::materials::detail::calculate_material_impl(mat, u, v, sprout::tuples::tuple_indexes<Material>::make()))
 			{
-				return sprout::tuples::make_tuple(
-					sprout::darkroom::materials::calculate(sprout::darkroom::materials::color(mat), u, v),
-					sprout::darkroom::materials::calculate(sprout::darkroom::materials::reflection(mat), u, v)
-					);
+				return sprout::darkroom::materials::detail::calculate_material_impl(mat, u, v, sprout::tuples::tuple_indexes<Material>::make());
 			}
 
 			//
 			// make_material_image
 			//
-			template<typename ColorImage, typename ReflectionImage>
-			inline SPROUT_CONSTEXPR sprout::tuples::tuple<ColorImage, ReflectionImage>
-			make_material_image(ColorImage const& col, ReflectionImage const& ref) {
-				return sprout::tuples::make_tuple(col, ref);
+			template<typename... Images>
+			inline SPROUT_CONSTEXPR sprout::tuples::tuple<Images...>
+			make_material_image(Images const&... images) {
+				return sprout::tuples::make_tuple(images...);
 			}
-
-			//
-			// material
-			//
-			typedef sprout::tuples::tuple<sprout::darkroom::colors::rgb_f_t, double> material;
 		}	// namespace materials
 	}	// namespace darkroom
 }	// namespace sprout
diff --git a/sprout/darkroom/materials/plaid.hpp b/sprout/darkroom/materials/plaid.hpp
index 7e7b4f35..207826dc 100644
--- a/sprout/darkroom/materials/plaid.hpp
+++ b/sprout/darkroom/materials/plaid.hpp
@@ -11,6 +11,9 @@
 #include <cstdint>
 #include <sprout/config.hpp>
 #include <sprout/tuple/tuple.hpp>
+#include <sprout/index_tuple/make_index_tuple.hpp>
+#include <sprout/utility/pack.hpp>
+#include <sprout/type_traits/enabler_if.hpp>
 #include <sprout/math/fmod.hpp>
 
 namespace sprout {
@@ -74,36 +77,56 @@ namespace sprout {
 			//
 			// make_plaid_material_image
 			//
-			template<typename Color, typename Reflection>
-			inline SPROUT_CONSTEXPR sprout::tuples::tuple<
-				sprout::darkroom::materials::plaid_element<Color>,
-				sprout::darkroom::materials::plaid_element<Reflection>
+			namespace detail {
+				template<typename... Elements, sprout::index_t... Indexes>
+				inline SPROUT_CONSTEXPR auto
+				make_plaid_material_image_impl(
+					sprout::index_tuple<Indexes...>,
+					Elements const&... elems
+					)
+				-> decltype(sprout::tuples::make_tuple(
+					sprout::darkroom::materials::make_plaid(sprout::pack_get<Indexes * 2>(elems...), sprout::pack_get<Indexes * 2 + 1>(elems...))...
+					))
+				{
+					return sprout::tuples::make_tuple(
+						sprout::darkroom::materials::make_plaid(sprout::pack_get<Indexes * 2>(elems...), sprout::pack_get<Indexes * 2 + 1>(elems...))...
+						);
+				}
+				template<typename Unit, typename... Elements, sprout::index_t... Indexes>
+				inline SPROUT_CONSTEXPR auto
+				make_plaid_material_image_impl(
+					Unit const& scale,
+					sprout::index_tuple<Indexes...>,
+					Elements const&... elems
+					)
+				-> decltype(sprout::tuples::make_tuple(
+					sprout::darkroom::materials::make_plaid(sprout::pack_get<Indexes * 2>(elems...), sprout::pack_get<Indexes * 2 + 1>(elems...), scale)...
+					))
+				{
+					return sprout::tuples::make_tuple(
+						sprout::darkroom::materials::make_plaid(sprout::pack_get<Indexes * 2>(elems...), sprout::pack_get<Indexes * 2 + 1>(elems...), scale)...
+						);
+				}
+			}	// namespace detail
+			template<
+				typename... Elements,
+				typename sprout::enabler_if<sizeof...(Elements) % 2 == 0>::type = sprout::enabler
 			>
-			make_plaid_material_image(
-				Color const& col1, Color const& col2,
-				Reflection const& ref1, Reflection const& ref2
-				)
+			inline SPROUT_CONSTEXPR auto
+			make_plaid_material_image(Elements const&... elems)
+			-> decltype(sprout::darkroom::materials::detail::make_plaid_material_image_impl(sprout::make_index_tuple<sizeof...(Elements) / 2>::make(), elems...))
 			{
-				return sprout::tuples::make_tuple(
-					sprout::darkroom::materials::make_plaid(col1, col2),
-					sprout::darkroom::materials::make_plaid(ref1, ref2)
-					);
+				return sprout::darkroom::materials::detail::make_plaid_material_image_impl(sprout::make_index_tuple<sizeof...(Elements) / 2>::make(), elems...);
 			}
-			template<typename Color, typename Reflection, typename Unit>
-			inline SPROUT_CONSTEXPR sprout::tuples::tuple<
-				sprout::darkroom::materials::plaid_element<Color, Unit>,
-				sprout::darkroom::materials::plaid_element<Reflection, Unit>
+			template<
+				typename Unit, typename... Elements,
+				typename sprout::enabler_if<sizeof...(Elements) % 2 == 0>::type = sprout::enabler
 			>
-			make_plaid_material_image(
-				Color const& col1, Color const& col2,
-				Reflection const& ref1, Reflection const& ref2,
-				Unit const& scale
-				)
+			inline SPROUT_CONSTEXPR auto
+			make_plaid_material_image(Unit const& scale, Elements const&... elems)
+			-> decltype(sprout::darkroom::materials::detail::make_plaid_material_image_impl(scale, sprout::make_index_tuple<sizeof...(Elements) / 2>::make(), elems...))
 			{
-				return sprout::tuples::make_tuple(
-					sprout::darkroom::materials::make_plaid(col1, col2, scale),
-					sprout::darkroom::materials::make_plaid(ref1, ref2, scale)
-					);
+				return sprout::darkroom::materials::detail::make_plaid_material_image_impl(scale, sprout::make_index_tuple<sizeof...(Elements) / 2>::make(), elems...);
 			}
 		}	// namespace materials
 	}	// namespace darkroom
diff --git a/sprout/darkroom/materials/uniform.hpp b/sprout/darkroom/materials/uniform.hpp
index 5f634abd..ec93bb87 100644
--- a/sprout/darkroom/materials/uniform.hpp
+++ b/sprout/darkroom/materials/uniform.hpp
@@ -44,15 +44,13 @@ namespace sprout {
 			//
 			// make_uniform_material_image
 			//
-			template<typename Color, typename Reflection>
+			template<typename... Elements>
 			inline SPROUT_CONSTEXPR sprout::tuples::tuple<
-				sprout::darkroom::materials::uniform_element<Color>,
-				sprout::darkroom::materials::uniform_element<Reflection>
+				sprout::darkroom::materials::uniform_element<Elements>...
 			>
-			make_uniform_material_image(Color const& col, Reflection const& ref) {
+			make_uniform_material_image(Elements const&... elems) {
 				return sprout::tuples::make_tuple(
-					sprout::darkroom::materials::make_uniform(col),
-					sprout::darkroom::materials::make_uniform(ref)
+					sprout::darkroom::materials::make_uniform(elems)...
 					);
 			}
 		}	// namespace materials
diff --git a/sprout/utility/pack.hpp b/sprout/utility/pack.hpp
index 81098d19..c6ace6ce 100644
--- a/sprout/utility/pack.hpp
+++ b/sprout/utility/pack.hpp
@@ -40,10 +40,10 @@ namespace sprout {
 		struct pack_get_helper;
 		template<typename... Args>
 		struct pack_get_helper<sprout::types::type_tuple<Args...> > {
-			template<typename T>
-			static SPROUT_CONSTEXPR T&&
-			eval(Args&&..., T&& t, ...) {
-				return sprout::forward<T>(t);
+			template<typename Head, typename... Tail>
+			static SPROUT_CONSTEXPR Head&&
+			eval(Args&&..., Head&& head, Tail&&...) {
+				return sprout::forward<Head>(head);
 			}
 		};
 	}	// namespace detail