From 4eb10e814756020a0618b1cc0056a44a75132a0b Mon Sep 17 00:00:00 2001
From: Nana Sakisaka <1901813+saki7@users.noreply.github.com>
Date: Wed, 23 Sep 2026 19:49:21 +0900
Subject: [PATCH] Split `type_list` and pack indexing to a separate header

---
 include/iris/alloy/adapt.hpp                  |   2 +-
 .../detail/preprocessed/tuple_impl.hpp.pre.in |   2 +-
 include/iris/alloy/detail/std_get.hpp         |   2 +-
 include/iris/alloy/detail/tuple_impl.hpp      |   2 +-
 include/iris/alloy/traits.hpp                 |   4 +-
 include/iris/alloy/utility.hpp                |   2 +-
 include/iris/hash.hpp                         |   3 +-
 .../iris/rvariant/detail/variant_storage.hpp  |  86 +++----
 include/iris/rvariant/detail/visit.hpp        |  19 +-
 include/iris/rvariant/pack.hpp                |   6 +-
 include/iris/rvariant/rvariant.hpp            |   1 +
 include/iris/rvariant/subset.hpp              |   4 +-
 include/iris/rvariant/variant_helper.hpp      |   3 +-
 include/iris/type_list.hpp                    | 231 ++++++++++++++++++
 include/iris/type_traits.hpp                  | 183 +-------------
 include/iris/units/traits.hpp                 |   1 +
 test/CMakeLists.txt                           |   1 +
 test/core.cpp                                 |  42 ----
 test/rvariant/get_visit.cpp                   |   2 +-
 test/type_list.cpp                            |  68 ++++++
 test/type_traits.cpp                          |   8 +-
 21 files changed, 380 insertions(+), 292 deletions(-)
 create mode 100644 include/iris/type_list.hpp
 create mode 100644 test/type_list.cpp

diff --git a/include/iris/alloy/adapt.hpp b/include/iris/alloy/adapt.hpp
index f4f7031..2dfde30 100644
--- a/include/iris/alloy/adapt.hpp
+++ b/include/iris/alloy/adapt.hpp
@@ -5,7 +5,7 @@
 
 #include <iris/config.hpp> // IWYU pragma: keep
 
-#include <iris/type_traits.hpp> // IWYU pragma: keep
+#include <iris/type_list.hpp> // IWYU pragma: keep
 
 #include <iris/pp/comma.hpp>
 #include <iris/pp/seq.hpp>
diff --git a/include/iris/alloy/detail/preprocessed/tuple_impl.hpp.pre.in b/include/iris/alloy/detail/preprocessed/tuple_impl.hpp.pre.in
index 2c723e5..347f65e 100644
--- a/include/iris/alloy/detail/preprocessed/tuple_impl.hpp.pre.in
+++ b/include/iris/alloy/detail/preprocessed/tuple_impl.hpp.pre.in
@@ -9,7 +9,7 @@
 
 #include <iris/alloy/traits.hpp>
 
-#include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
 
 #include <type_traits>
 
diff --git a/include/iris/alloy/detail/std_get.hpp b/include/iris/alloy/detail/std_get.hpp
index cc149af..d571087 100644
--- a/include/iris/alloy/detail/std_get.hpp
+++ b/include/iris/alloy/detail/std_get.hpp
@@ -5,7 +5,7 @@
 
 #include <iris/config.hpp> // IWYU pragma: keep
 
-#include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
 
 #include <utility>
 
diff --git a/include/iris/alloy/detail/tuple_impl.hpp b/include/iris/alloy/detail/tuple_impl.hpp
index 4100180..1624c89 100644
--- a/include/iris/alloy/detail/tuple_impl.hpp
+++ b/include/iris/alloy/detail/tuple_impl.hpp
@@ -11,7 +11,7 @@
 
 #include <iris/alloy/traits.hpp>
 
-#include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
 
 #include <type_traits>
 
diff --git a/include/iris/alloy/traits.hpp b/include/iris/alloy/traits.hpp
index 3e66e42..3d9a379 100644
--- a/include/iris/alloy/traits.hpp
+++ b/include/iris/alloy/traits.hpp
@@ -7,12 +7,12 @@
 
 #include <iris/alloy/detail/deduce.hpp>
 
-#include <iris/type_traits.hpp>
+#include <iris/bits/specialization_of.hpp>
 
 #include <functional>
 #include <type_traits>
 
-#include <cstddef>
+#include <cstddef> // IWYU pragma: keep
 
 namespace iris::alloy {
 
diff --git a/include/iris/alloy/utility.hpp b/include/iris/alloy/utility.hpp
index d757202..8d4a284 100644
--- a/include/iris/alloy/utility.hpp
+++ b/include/iris/alloy/utility.hpp
@@ -9,7 +9,7 @@
 #include <iris/alloy/tuple.hpp>
 
 #include <iris/utility.hpp>
-#include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
 
 #include <functional> // std::invoke
 #include <type_traits>
diff --git a/include/iris/hash.hpp b/include/iris/hash.hpp
index 30ce008..ad9cead 100644
--- a/include/iris/hash.hpp
+++ b/include/iris/hash.hpp
@@ -6,7 +6,8 @@
 #include <iris/config.hpp> // IWYU pragma: keep
 
 #include <iris/requirements.hpp>
-#include <iris/type_traits.hpp>
+#include <iris/bits/specialization_of.hpp>
+#include <iris/bits/is_function_object.hpp>
 
 #include <functional>
 #include <ranges>
diff --git a/include/iris/rvariant/detail/variant_storage.hpp b/include/iris/rvariant/detail/variant_storage.hpp
index 04874f3..dc89dab 100644
--- a/include/iris/rvariant/detail/variant_storage.hpp
+++ b/include/iris/rvariant/detail/variant_storage.hpp
@@ -9,10 +9,10 @@
 
 #include <iris/rvariant/detail/rvariant_fwd.hpp>
 
-#include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
+#include <iris/bits/specialization_of.hpp>
 
 #include <type_traits>
-#include <utility>
 
 #include <cstddef> // IWYU pragma: keep
 
@@ -79,7 +79,7 @@ template<class Variant, class T>
 //   => valueless iff move constructor throws
 //
 // Emplace (if VT(Args...) is throwing)
-//   rvariant tmp(std::in_place_index<I>, std::forward<Args>(args)...);
+//   rvariant tmp(std::in_place_index<I>, static_cast<Args&&>(args)...);
 //   *this = std::move(tmp);
 //        ^^^ needs to be NOT observable on user's part, as per "Effects" https://eel.is/c++draft/variant.mod#7
 //                        ^^^^^^^^^^^^^^
@@ -193,7 +193,7 @@ IRIS_RVARIANT_ALWAYS_THROWING_UNREACHABLE_BEGIN
         requires std::is_constructible_v<T, Args...> // required for not confusing some compilers
     constexpr explicit variadic_union(std::in_place_index_t<0>, Args&&... args)
         noexcept(std::is_nothrow_constructible_v<T, Args...>)
-        : first(std::forward<Args>(args)...) // value-initialize; https://eel.is/c++draft/variant.ctor#3
+        : first(static_cast<Args&&>(args)...) // value-initialize; https://eel.is/c++draft/variant.ctor#3
     {}
 IRIS_RVARIANT_ALWAYS_THROWING_UNREACHABLE_END
 
@@ -203,7 +203,7 @@ IRIS_RVARIANT_ALWAYS_THROWING_UNREACHABLE_END
             std::is_constructible_v<IRIS_PACK_INDEXING(I - 1, Ts...), Args...>
     constexpr explicit variadic_union(std::in_place_index_t<I>, Args&&... args)
         noexcept(std::is_nothrow_constructible_v<IRIS_PACK_INDEXING(I - 1, Ts...), Args...>)
-        : rest(std::in_place_index<I - 1>, std::forward<Args>(args)...)
+        : rest(std::in_place_index<I - 1>, static_cast<Args&&>(args)...)
     {}
 
     union {
@@ -245,7 +245,7 @@ IRIS_RVARIANT_ALWAYS_THROWING_UNREACHABLE_BEGIN
         requires std::is_constructible_v<T, Args...> // required for not confusing some compilers
     constexpr explicit variadic_union(std::in_place_index_t<0>, Args&&... args)
         noexcept(std::is_nothrow_constructible_v<T, Args...>)
-        : first(std::forward<Args>(args)...) // value-initialize; https://eel.is/c++draft/variant.ctor#3
+        : first(static_cast<Args&&>(args)...) // value-initialize; https://eel.is/c++draft/variant.ctor#3
     {}
 IRIS_RVARIANT_ALWAYS_THROWING_UNREACHABLE_END
 
@@ -255,7 +255,7 @@ IRIS_RVARIANT_ALWAYS_THROWING_UNREACHABLE_END
             std::is_constructible_v<IRIS_PACK_INDEXING(I - 1, Ts...), Args...>
     constexpr explicit variadic_union(std::in_place_index_t<I>, Args&&... args)
         noexcept(std::is_nothrow_constructible_v<IRIS_PACK_INDEXING(I - 1, Ts...), Args...>)
-        : rest(std::in_place_index<I - 1>, std::forward<Args>(args)...)
+        : rest(std::in_place_index<I - 1>, static_cast<Args&&>(args)...)
     {}
 
     union {
@@ -281,56 +281,56 @@ template<class Variant>
 [[nodiscard]] IRIS_FORCEINLINE constexpr forward_storage_t<Variant>&&
 forward_storage(std::remove_reference_t<Variant>& v IRIS_LIFETIMEBOUND) noexcept
 {
-    return std::forward<Variant>(v).storage();
+    return static_cast<Variant&&>(v).storage();
 }
 
 template<class Variant>
 [[nodiscard]] IRIS_FORCEINLINE constexpr forward_storage_t<Variant>&&
 forward_storage(std::remove_reference_t<Variant>&& v IRIS_LIFETIMEBOUND) noexcept  // NOLINT(cppcoreguidelines-rvalue-reference-param-not-moved)
 {
-    return std::forward<Variant>(v).storage();
+    return static_cast<Variant&&>(v).storage();
 }
 
 
 template<std::size_t I, class Storage>
 [[nodiscard]] IRIS_FORCEINLINE constexpr auto&& raw_get(Storage&& storage IRIS_LIFETIMEBOUND) noexcept
 {
-         if constexpr (I ==  0) return std::forward<Storage>(storage).first;
-    else if constexpr (I ==  1) return std::forward<Storage>(storage).rest.first;
-    else if constexpr (I ==  2) return std::forward<Storage>(storage).rest.rest.first;
-    else if constexpr (I ==  3) return std::forward<Storage>(storage).rest.rest.rest.first;
-    else if constexpr (I ==  4) return std::forward<Storage>(storage).rest.rest.rest.rest.first;
-    else if constexpr (I ==  5) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.first;
-    else if constexpr (I ==  6) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I ==  7) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I ==  8) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I ==  9) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 10) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 11) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 12) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 13) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 14) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 15) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 16) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 17) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 18) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 19) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 20) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 21) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 22) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 23) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 24) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 25) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 26) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 27) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 28) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 29) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 30) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
-    else if constexpr (I == 31) return std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+         if constexpr (I ==  0) return static_cast<Storage&&>(storage).first;
+    else if constexpr (I ==  1) return static_cast<Storage&&>(storage).rest.first;
+    else if constexpr (I ==  2) return static_cast<Storage&&>(storage).rest.rest.first;
+    else if constexpr (I ==  3) return static_cast<Storage&&>(storage).rest.rest.rest.first;
+    else if constexpr (I ==  4) return static_cast<Storage&&>(storage).rest.rest.rest.rest.first;
+    else if constexpr (I ==  5) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.first;
+    else if constexpr (I ==  6) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I ==  7) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I ==  8) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I ==  9) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 10) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 11) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 12) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 13) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 14) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 15) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 16) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 17) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 18) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 19) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 20) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 21) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 22) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 23) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 24) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 25) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 26) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 27) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 28) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 29) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 30) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
+    else if constexpr (I == 31) return static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.first;
     else if constexpr (I < 64)  return detail::raw_get<I - 32>(
-                                       std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest);
+                                       static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest);
     else                        return detail::raw_get<I - 64>(
-                                       std::forward<Storage>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest
+                                       static_cast<Storage&&>(storage).rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest
                                                                      .rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest.rest);
 }
 
diff --git a/include/iris/rvariant/detail/visit.hpp b/include/iris/rvariant/detail/visit.hpp
index 41a2f38..b003d53 100644
--- a/include/iris/rvariant/detail/visit.hpp
+++ b/include/iris/rvariant/detail/visit.hpp
@@ -12,6 +12,7 @@
 #include <iris/rvariant/detail/variant_storage.hpp>
 #include <iris/rvariant/variant_helper.hpp>
 
+#include <iris/type_list.hpp>
 #include <iris/type_traits.hpp>
 
 #include <variant> // std::bad_variant_access
@@ -108,11 +109,11 @@ do_raw_visit(Visitor&& vis, Storage&& storage)  // NOLINT(cppcoreguidelines-rval
     noexcept(raw_visit_noexcept<I, Visitor, Storage>::value)
 {
     if constexpr (!std::remove_cvref_t<Storage>::never_valueless && I == 0) {
-        return std::forward<Visitor>(vis)(std::in_place_index<std::variant_npos>, std::forward<Storage>(storage));
+        return static_cast<Visitor&&>(vis)(std::in_place_index<std::variant_npos>, static_cast<Storage&&>(storage));
 
     } else {
         constexpr std::size_t RealI = valueless_unbias<Storage>(I);
-        return std::forward<Visitor>(vis)(std::in_place_index<RealI>, raw_get<RealI>(std::forward<Storage>(storage)));
+        return static_cast<Visitor&&>(vis)(std::in_place_index<RealI>, detail::raw_get<RealI>(static_cast<Storage&&>(storage)));
     }
 }
 
@@ -170,7 +171,7 @@ struct raw_visit_dispatch<NeverValueless, -1>
     {
         constexpr auto const& table = raw_visit_table<Visitor, Storage>::table;
         auto const& f = table[i];
-        return f(std::forward<Visitor>(vis), std::forward<Storage>(storage));
+        return f(static_cast<Visitor&&>(vis), static_cast<Storage&&>(storage));
     }
 };
 
@@ -247,7 +248,7 @@ raw_visit(Variant&& v, Visitor&& vis)  // NOLINT(cppcoreguidelines-missing-std-f
         N, Visitor, forward_storage_t<Variant>
     >(
         detail::valueless_bias<Variant>(v.index_),
-        std::forward<Visitor>(vis),
+        static_cast<Visitor&&>(vis),
         detail::forward_storage<Variant>(v)
     );
 }
@@ -262,7 +263,7 @@ raw_visit_i(std::size_t const biased_i, Variant&& v, Visitor&& vis)  // NOLINT(c
         N, Visitor, forward_storage_t<Variant>
     >(
         biased_i,
-        std::forward<Visitor>(vis),
+        static_cast<Visitor&&>(vis),
         detail::forward_storage<Variant>(v)
     );
 }
@@ -415,9 +416,9 @@ struct multi_visitor<std::index_sequence<Is...>>
 
         } else {
             return std::invoke_r<R>(
-                std::forward<Visitor>(vis),
+                static_cast<Visitor&&>(vis),
                 unwrap_recursive(
-                    raw_get<valueless_unbias<Storage>(Is)>(std::forward<Storage>(storage))
+                    detail::raw_get<valueless_unbias<Storage>(Is)>(static_cast<Storage&&>(storage))
                 )...
             );
         }
@@ -455,7 +456,7 @@ struct visit_dispatch<-1>
     {
         constexpr auto const& table = visit_table<R, OverloadSeq, Visitor, Storage...>::table;
         auto const& f = table[flat_i];
-        return f(std::forward<Visitor>(vis), std::forward<Storage>(storage)...);
+        return f(static_cast<Visitor&&>(vis), static_cast<Storage&&>(storage)...);
     }
 };
 
@@ -587,7 +588,7 @@ struct visit_impl<
         return visit_dispatch<visit_strategy<OverloadSeq::size>>::template apply<
             R, OverloadSeq, Visitor, forward_storage_t<as_variant_t<Variants>>...
         >(
-            flat_i, std::forward<Visitor>(vis), forward_storage<as_variant_t<Variants>>(vars)...
+            flat_i, static_cast<Visitor&&>(vis), detail::forward_storage<as_variant_t<Variants>>(vars)...
         );
     }
 };
diff --git a/include/iris/rvariant/pack.hpp b/include/iris/rvariant/pack.hpp
index 5640bd3..db8b2fe 100644
--- a/include/iris/rvariant/pack.hpp
+++ b/include/iris/rvariant/pack.hpp
@@ -7,7 +7,9 @@
 
 // Utilities related to [rvariant.pack]
 
-#include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
+
+#include <type_traits>
 
 namespace iris {
 
@@ -25,7 +27,7 @@ struct pack_union_impl<TT, type_list<Ts...>>
 template<template<class...> class TT, class... Ts, class U, class... Us>
 struct pack_union_impl<TT, type_list<Ts...>, U, Us...>
     : std::conditional_t<
-    is_in_v<U, Ts...>,
+        is_in_v<U, Ts...>,
         pack_union_impl<TT, type_list<Ts...>, Us...>,
         pack_union_impl<TT, type_list<Ts..., U>, Us...>
     >
diff --git a/include/iris/rvariant/rvariant.hpp b/include/iris/rvariant/rvariant.hpp
index 1de4e30..44e67de 100644
--- a/include/iris/rvariant/rvariant.hpp
+++ b/include/iris/rvariant/rvariant.hpp
@@ -17,6 +17,7 @@
 #include <iris/compare.hpp>
 #include <iris/cond_trivial_smf.hpp>
 #include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
 #include <iris/hash.hpp>
 
 #include <concepts>
diff --git a/include/iris/rvariant/subset.hpp b/include/iris/rvariant/subset.hpp
index 758e4f5..d86dc04 100644
--- a/include/iris/rvariant/subset.hpp
+++ b/include/iris/rvariant/subset.hpp
@@ -7,7 +7,9 @@
 
 #include <iris/rvariant/detail/rvariant_fwd.hpp>
 #include <iris/rvariant/variant_helper.hpp>
-#include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
+
+#include <iris/bits/specialization_of.hpp>
 
 #include <type_traits>
 
diff --git a/include/iris/rvariant/variant_helper.hpp b/include/iris/rvariant/variant_helper.hpp
index 4baa6e6..7267371 100644
--- a/include/iris/rvariant/variant_helper.hpp
+++ b/include/iris/rvariant/variant_helper.hpp
@@ -12,7 +12,8 @@
 #include <iris/config.hpp> // IWYU pragma: keep
 
 #include <iris/rvariant/detail/rvariant_fwd.hpp>
-#include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
+#include <iris/bits/specialization_of.hpp>
 
 #include <utility>
 #include <type_traits>
diff --git a/include/iris/type_list.hpp b/include/iris/type_list.hpp
new file mode 100644
index 0000000..e495b69
--- /dev/null
+++ b/include/iris/type_list.hpp
@@ -0,0 +1,231 @@
+#ifndef IRIS_ZZ_TYPE_LIST_HPP
+#define IRIS_ZZ_TYPE_LIST_HPP
+
+// SPDX-License-Identifier: MIT
+
+#include <iris/config.hpp> // IWYU pragma: keep
+
+#include <type_traits>
+#include <utility>
+
+#include <cstddef> // IWYU pragma: keep
+
+namespace iris {
+
+template<class... Ts>
+struct type_list
+{
+    static constexpr std::size_t size = sizeof...(Ts);
+};
+
+template<auto... Vals>
+struct constant_list
+{
+    static constexpr std::size_t size = sizeof...(Vals);
+};
+
+// ----------------------------------------------------------
+
+template<class... Lists>
+struct concat_type_list;
+
+// `concat_type_list_t` is intentionally not defined; typical use case
+// of concatenating type lists involves deep recursive instantiation of
+// parent templates, which sometimes makes IDE's completion choke on
+// single additional type alias of `_t`. Just use `typename ...::type`.
+
+template<>
+struct concat_type_list<>
+{
+    using type = type_list<>;
+};
+
+template<class... Ts>
+struct concat_type_list<type_list<Ts...>>
+{
+    using type = type_list<Ts...>;
+};
+
+template<class... Ts, class... Us>
+struct concat_type_list<type_list<Ts...>, type_list<Us...>>
+{
+    using type = type_list<Ts..., Us...>;
+};
+
+template<class... Ts, class... Us, class... Rest>
+struct concat_type_list<type_list<Ts...>, type_list<Us...>, Rest...>
+    : concat_type_list<type_list<Ts..., Us...>, Rest...>
+{};
+
+// ----------------------------------------------------------
+
+namespace detail {
+
+template<class Voids>
+struct do_pack_indexing;
+
+template<std::size_t... Voids>
+struct do_pack_indexing<std::index_sequence<Voids...>>
+{
+    template<class T>
+    static std::type_identity<T> select(
+        // ReSharper disable once CppCStyleCast
+        decltype((void*)Voids)...,
+        std::type_identity<T>*,
+        ...
+    );
+};
+
+template<class Voids>
+struct do_cpack_indexing;
+
+template<std::size_t... Voids>
+struct do_cpack_indexing<std::index_sequence<Voids...>>
+{
+    template<class T, T N>
+    static std::integral_constant<T, N> select(
+        // ReSharper disable once CppCStyleCast
+        decltype((void*)Voids)...,
+        std::integral_constant<T, N>*,
+        ...
+    );
+};
+
+} // detail
+
+
+template<std::size_t I, class T>
+struct at_c;
+
+template<std::size_t I, class T>
+using at_c_t = at_c<I, T>::type;
+
+// Has native pack indexing?
+// Note: GCC 15 emits "sorry, unimplemented: mangling type pack index"
+#if __cpp_pack_indexing >= 202311L && \
+    !(defined(__GNUC__) && !defined(__clang__) && __GNUC__ <= 15) && \
+    !defined(__clang__)
+
+# define IRIS_PACK_INDEXING(I, Ts_ellipsis) Ts_ellipsis[I]
+
+template<std::size_t I, class... Ts> struct pack_indexing { using type = Ts...[I]; };
+template<std::size_t I, class... Ts> using pack_indexing_t = Ts...[I];
+template<std::size_t I, auto... Ns> struct cpack_indexing { static constexpr auto value = Ns...[I]; };
+template<std::size_t I, auto... Ns> constexpr auto cpack_indexing_v = Ns...[I];
+
+template<std::size_t I, template<class...> class TT, class... Ts>
+struct at_c<I, TT<Ts...>>
+{
+    using type = Ts...[I];
+};
+
+// ----------------------------------------------------------
+#else // no native pack indexing
+template<std::size_t I, class... Ts>
+struct pack_indexing
+{
+    static_assert(I < sizeof...(Ts));
+    using type = decltype(detail::do_pack_indexing<std::make_index_sequence<I>>::select(
+        static_cast<std::type_identity<Ts>*>(nullptr)...
+    ))::type;
+};
+
+template<std::size_t I, class... Ts>
+using pack_indexing_t = pack_indexing<I, Ts...>::type;
+
+# define IRIS_PACK_INDEXING(I, Ts_ellipsis) ::iris::pack_indexing_t<I, Ts_ellipsis>
+
+
+template<std::size_t I, auto... Ns>
+struct cpack_indexing
+{
+    static_assert(I < sizeof...(Ns));
+    static constexpr auto value = decltype(detail::do_cpack_indexing<std::make_index_sequence<I>>::select(
+        static_cast<std::integral_constant<decltype(Ns), Ns>*>(nullptr)...
+    ))::value;
+};
+
+template<std::size_t I, auto... Ns>
+constexpr auto cpack_indexing_v = cpack_indexing<I, Ns...>::value;
+
+
+template<std::size_t I, template<class...> class TT, class... Ts>
+struct at_c<I, TT<Ts...>>
+{
+    static_assert(I < sizeof...(Ts));
+    using type = decltype(detail::do_pack_indexing<std::make_index_sequence<I>>::select(
+        static_cast<std::type_identity<Ts>*>(nullptr)...
+    ))::type;
+};
+#endif
+
+// ----------------------------------------------------------
+
+inline constexpr std::size_t find_npos = static_cast<std::size_t>(-1);
+
+namespace detail {
+
+template<std::size_t I, class T, class... Ts>
+struct find_index_impl
+    : std::integral_constant<std::size_t, find_npos>
+{};
+
+template<std::size_t I, class T, class U, class... Us>
+struct find_index_impl<I, T, U, Us...>
+    : std::conditional_t<
+        std::is_same_v<T, U>,
+        std::integral_constant<std::size_t, I>,
+        find_index_impl<I + 1, T, Us...>
+    >
+{};
+
+} // detail
+
+template<class T, class List>
+struct find_index;
+
+template<class T, template<class...> class TT, class... Ts>
+struct find_index<T, TT<Ts...>> : detail::find_index_impl<0, T, Ts...> {};
+
+template<class T, class List>
+inline constexpr std::size_t find_index_v = find_index<T, List>::value;
+
+
+template<class T, class... Ts>
+struct is_in : std::disjunction<std::is_same<T, Ts>...> {};
+
+template<class T, class... Ts>
+inline constexpr bool is_in_v = is_in<T, Ts...>::value;
+
+// ----------------------------------------------------------
+
+namespace detail {
+
+template<bool Found, class T, class... Us>
+struct exactly_once_impl : std::bool_constant<Found> {};
+
+template<class T, class U, class... Us>
+struct exactly_once_impl<false, T, U, Us...>
+    : exactly_once_impl<std::is_same_v<T, U>, T, Us...> {};
+
+template<class T, class U, class... Us>
+struct exactly_once_impl<true, T, U, Us...>
+    : std::conditional_t<std::is_same_v<T, U>, std::false_type, exactly_once_impl<true, T, Us...>> {};
+
+} // detail
+
+template<class T, class List>
+struct exactly_once;
+
+template<class T, template<class...> class TT, class... Ts>
+struct exactly_once<T, TT<Ts...>> : detail::exactly_once_impl<false, T, Ts...>
+{
+    static_assert(sizeof...(Ts) > 0);
+};
+
+template<class T, class List>
+inline constexpr bool exactly_once_v = exactly_once<T, List>::value;
+
+} // iris
+
+#endif
diff --git a/include/iris/type_traits.hpp b/include/iris/type_traits.hpp
index 058d0dc..05098f5 100644
--- a/include/iris/type_traits.hpp
+++ b/include/iris/type_traits.hpp
@@ -146,158 +146,6 @@ struct symmetric_common_type<Ts...> : dominant_type<Ts...>
 
 // -----------------------------------------------------------
 
-template<class... Ts>
-struct type_list
-{
-    static constexpr std::size_t size = sizeof...(Ts);
-};
-
-template<auto... Vals>
-struct constant_list
-{
-    static constexpr std::size_t size = sizeof...(Vals);
-};
-
-namespace detail {
-
-template<class Voids>
-struct do_pack_indexing;
-
-template<std::size_t... Voids>
-struct do_pack_indexing<std::index_sequence<Voids...>>
-{
-    template<class T>
-    static std::type_identity<T> select(
-        // ReSharper disable once CppCStyleCast
-        decltype((void*)Voids)...,
-        std::type_identity<T>*,
-        ...
-    );
-};
-
-template<class Voids>
-struct do_cpack_indexing;
-
-template<std::size_t... Voids>
-struct do_cpack_indexing<std::index_sequence<Voids...>>
-{
-    template<class T, T N>
-    static std::integral_constant<T, N> select(
-        // ReSharper disable once CppCStyleCast
-        decltype((void*)Voids)...,
-        std::integral_constant<T, N>*,
-        ...
-    );
-};
-
-} // detail
-
-
-template<std::size_t I, class T>
-struct at_c;
-
-template<std::size_t I, class T>
-using at_c_t = at_c<I, T>::type;
-
-// Has native pack indexing?
-// Note: GCC 15 emits "sorry, unimplemented: mangling type pack index"
-#if __cpp_pack_indexing >= 202311L && \
-    !(defined(__GNUC__) && !defined(__clang__) && __GNUC__ <= 15) && \
-    !defined(__clang__)
-
-# define IRIS_PACK_INDEXING(I, Ts_ellipsis) Ts_ellipsis[I]
-
-template<std::size_t I, class... Ts> struct pack_indexing { using type = Ts...[I]; };
-template<std::size_t I, class... Ts> using pack_indexing_t = Ts...[I];
-template<std::size_t I, auto... Ns> struct cpack_indexing { static constexpr auto value = Ns...[I]; };
-template<std::size_t I, auto... Ns> constexpr auto cpack_indexing_v = Ns...[I];
-
-template<std::size_t I, template<class...> class TT, class... Ts>
-struct at_c<I, TT<Ts...>>
-{
-    using type = Ts...[I];
-};
-
-// ----------------------------------------------------------
-#else // no native pack indexing
-template<std::size_t I, class... Ts>
-struct pack_indexing
-{
-    static_assert(I < sizeof...(Ts));
-    using type = decltype(detail::do_pack_indexing<std::make_index_sequence<I>>::select(
-        static_cast<std::type_identity<Ts>*>(nullptr)...
-    ))::type;
-};
-
-template<std::size_t I, class... Ts>
-using pack_indexing_t = pack_indexing<I, Ts...>::type;
-
-# define IRIS_PACK_INDEXING(I, Ts_ellipsis) ::iris::pack_indexing_t<I, Ts_ellipsis>
-
-
-template<std::size_t I, auto... Ns>
-struct cpack_indexing
-{
-    static_assert(I < sizeof...(Ns));
-    static constexpr auto value = decltype(detail::do_cpack_indexing<std::make_index_sequence<I>>::select(
-        static_cast<std::integral_constant<decltype(Ns), Ns>*>(nullptr)...
-    ))::value;
-};
-
-template<std::size_t I, auto... Ns>
-constexpr auto cpack_indexing_v = cpack_indexing<I, Ns...>::value;
-
-
-template<std::size_t I, template<class...> class TT, class... Ts>
-struct at_c<I, TT<Ts...>>
-{
-    static_assert(I < sizeof...(Ts));
-    using type = decltype(detail::do_pack_indexing<std::make_index_sequence<I>>::select(
-        static_cast<std::type_identity<Ts>*>(nullptr)...
-    ))::type;
-};
-#endif
-
-// ----------------------------------------------------------
-
-
-inline constexpr std::size_t find_npos = static_cast<std::size_t>(-1);
-
-namespace detail {
-
-template<std::size_t I, class T, class... Ts>
-struct find_index_impl
-    : std::integral_constant<std::size_t, find_npos>
-{};
-
-template<std::size_t I, class T, class U, class... Us>
-struct find_index_impl<I, T, U, Us...>
-    : std::conditional_t<
-        std::is_same_v<T, U>,
-        std::integral_constant<std::size_t, I>,
-        find_index_impl<I + 1, T, Us...>
-    >
-{};
-
-} // detail
-
-template<class T, class List>
-struct find_index;
-
-template<class T, template<class...> class TT, class... Ts>
-struct find_index<T, TT<Ts...>> : detail::find_index_impl<0, T, Ts...> {};
-
-template<class T, class List>
-inline constexpr std::size_t find_index_v = find_index<T, List>::value;
-
-
-template<class T, class... Ts>
-struct is_in : std::disjunction<std::is_same<T, Ts>...> {};
-
-template<class T, class... Ts>
-inline constexpr bool is_in_v = is_in<T, Ts...>::value;
-
-
 template<template<class A, class B> class F, class U, class... Ts>
 struct disjunction_for : std::disjunction<F<U, Ts>...> {};
 
@@ -310,34 +158,7 @@ struct conjunction_for : std::conjunction<F<U, Ts>...> {};
 template<template<class A, class B> class F, class U, class... Ts>
 inline constexpr bool conjunction_for_v = conjunction_for<F, U, Ts...>::value;
 
-
-namespace detail {
-
-template<bool Found, class T, class... Us>
-struct exactly_once_impl : std::bool_constant<Found> {};
-
-template<class T, class U, class... Us>
-struct exactly_once_impl<false, T, U, Us...>
-    : exactly_once_impl<std::is_same_v<T, U>, T, Us...> {};
-
-template<class T, class U, class... Us>
-struct exactly_once_impl<true, T, U, Us...>
-    : std::conditional_t<std::is_same_v<T, U>, std::false_type, exactly_once_impl<true, T, Us...>> {};
-
-} // detail
-
-template<class T, class List>
-struct exactly_once;
-
-template<class T, template<class...> class TT, class... Ts>
-struct exactly_once<T, TT<Ts...>> : detail::exactly_once_impl<false, T, Ts...>
-{
-    static_assert(sizeof...(Ts) > 0);
-};
-
-template<class T, class List>
-inline constexpr bool exactly_once_v = exactly_once<T, List>::value;
-
+// -----------------------------------------------------------
 
 // Provides definition equivalent to MSVC's STL for semantic compatibility
 namespace detail::has_ADL_swap_detail {
@@ -380,7 +201,7 @@ struct is_convertible_without_narrowing_array_check
 
 template<class From, class To>
     requires requires (From&& x) {
-        { std::type_identity_t<To[]>{std::forward<From>(x)} } -> std::same_as<To[1]>;
+        { std::type_identity_t<To[]>{static_cast<From&&>(x)} } -> std::same_as<To[1]>;
     }
 struct is_convertible_without_narrowing_array_check<From, To>
     : std::true_type
diff --git a/include/iris/units/traits.hpp b/include/iris/units/traits.hpp
index 79645a0..2193de1 100644
--- a/include/iris/units/traits.hpp
+++ b/include/iris/units/traits.hpp
@@ -8,6 +8,7 @@
 // Don't include <iris/units/concepts.hpp>; dependency ordering is important
 
 #include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
 
 #include <type_traits>
 #include <concepts>
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 403b218..a03ca6b 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -181,6 +181,7 @@ if(PROJECT_IS_TOP_LEVEL)
             IRIS_TEST_IRIS_TESTS
             library_config
             core
+            type_list
             type_traits
             ranges
             container_traits
diff --git a/test/core.cpp b/test/core.cpp
index c785453..c0476af 100644
--- a/test/core.cpp
+++ b/test/core.cpp
@@ -63,48 +63,6 @@ TEST_CASE("EBO")
     STATIC_CHECK(sizeof(Derived5_fixed) == 4);
 }
 
-TEST_CASE("pack_indexing")
-{
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<0, int>, int>);
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<0, int, float>, int>);
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<1, int, float>, float>);
-
-    // make sure non-object and non-referenceable type is working
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<0, void>, void>);
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<0, void, int>, void>);
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<1, void, int>, int>);
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<0, int, void>, int>);
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<1, int, void>, void>);
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<0, void, void>, void>);
-    STATIC_REQUIRE(std::is_same_v<iris::pack_indexing_t<1, void, void>, void>);
-
-    STATIC_REQUIRE(std::is_same_v<iris::at_c_t<0, iris::type_list<int>>, int>);
-    STATIC_REQUIRE(std::is_same_v<iris::at_c_t<0, iris::type_list<int, float>>, int>);
-    STATIC_REQUIRE(std::is_same_v<iris::at_c_t<1, iris::type_list<int, float>>, float>);
-}
-
-TEST_CASE("exactly_once")
-{
-    STATIC_REQUIRE(iris::exactly_once_v<int, iris::type_list<int, float>>);
-    STATIC_REQUIRE_FALSE(iris::exactly_once_v<int, iris::type_list<int, int>>);
-}
-
-TEST_CASE("is_in")
-{
-    STATIC_REQUIRE(iris::is_in_v<int, int, float>);
-    STATIC_REQUIRE_FALSE(iris::is_in_v<int, float>);
-}
-
-TEST_CASE("find_index")
-{
-    STATIC_REQUIRE(iris::find_index_v<int,    iris::type_list<int, float, double>> == 0);
-    STATIC_REQUIRE(iris::find_index_v<float,  iris::type_list<int, float, double>> == 1);
-    STATIC_REQUIRE(iris::find_index_v<double, iris::type_list<int, float, double>> == 2);
-    STATIC_REQUIRE(iris::find_index_v<int,    iris::type_list<float, double>> == iris::find_npos);
-
-    STATIC_REQUIRE(iris::find_index_v<int, iris::type_list<int, int, double>> == 0);
-}
-
 TEST_CASE("Cpp17EqualityComparable")
 {
     STATIC_REQUIRE(iris::req::Cpp17EqualityComparable<int>);
diff --git a/test/rvariant/get_visit.cpp b/test/rvariant/get_visit.cpp
index 08871e5..6bf130d 100644
--- a/test/rvariant/get_visit.cpp
+++ b/test/rvariant/get_visit.cpp
@@ -3,7 +3,7 @@
 #include "iris_rvariant_test.hpp"
 
 #include <iris/rvariant.hpp>
-#include <iris/type_traits.hpp>
+#include <iris/type_list.hpp>
 
 #include <concepts>
 #include <string_view>
diff --git a/test/type_list.cpp b/test/type_list.cpp
new file mode 100644
index 0000000..ac5de05
--- /dev/null
+++ b/test/type_list.cpp
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: MIT
+
+#include "iris_test.hpp"
+
+#include <iris/type_list.hpp>
+
+#include <concepts>
+
+using iris::type_list;
+using iris::concat_type_list;
+
+TEST_CASE("type_list")
+{
+    STATIC_CHECK(std::same_as<concat_type_list<>::type, type_list<>>);
+    STATIC_CHECK(std::same_as<concat_type_list<type_list<>>::type, type_list<>>);
+    STATIC_CHECK(std::same_as<concat_type_list<type_list<>, type_list<>>::type, type_list<>>);
+    STATIC_CHECK(std::same_as<concat_type_list<type_list<>, type_list<>, type_list<>>::type, type_list<>>);
+
+    STATIC_CHECK(std::same_as<concat_type_list<type_list<int>, type_list<>>::type, type_list<int>>);
+    STATIC_CHECK(std::same_as<concat_type_list<type_list<>, type_list<int>>::type, type_list<int>>);
+    STATIC_CHECK(std::same_as<concat_type_list<type_list<int>, type_list<int>>::type, type_list<int, int>>);
+
+    STATIC_CHECK(std::same_as<concat_type_list<type_list<int>, type_list<>, type_list<>>::type, type_list<int>>);
+    STATIC_CHECK(std::same_as<concat_type_list<type_list<>, type_list<int>, type_list<>>::type, type_list<int>>);
+    STATIC_CHECK(std::same_as<concat_type_list<type_list<>, type_list<>, type_list<int>>::type, type_list<int>>);
+}
+
+TEST_CASE("pack_indexing")
+{
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<0, int>, int>);
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<0, int, float>, int>);
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<1, int, float>, float>);
+
+    // make sure non-object and non-referenceable type is working
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<0, void>, void>);
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<0, void, int>, void>);
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<1, void, int>, int>);
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<0, int, void>, int>);
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<1, int, void>, void>);
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<0, void, void>, void>);
+    STATIC_CHECK(std::same_as<iris::pack_indexing_t<1, void, void>, void>);
+
+    STATIC_CHECK(std::same_as<iris::at_c_t<0, type_list<int>>, int>);
+    STATIC_CHECK(std::same_as<iris::at_c_t<0, type_list<int, float>>, int>);
+    STATIC_CHECK(std::same_as<iris::at_c_t<1, type_list<int, float>>, float>);
+}
+
+TEST_CASE("exactly_once")
+{
+    STATIC_CHECK(iris::exactly_once_v<int, type_list<int, float>>);
+    STATIC_CHECK_FALSE(iris::exactly_once_v<int, type_list<int, int>>);
+}
+
+TEST_CASE("is_in")
+{
+    STATIC_CHECK(iris::is_in_v<int, int, float>);
+    STATIC_CHECK(!iris::is_in_v<int, float>);
+}
+
+TEST_CASE("find_index")
+{
+    STATIC_CHECK(iris::find_index_v<int,    type_list<int, float, double>> == 0);
+    STATIC_CHECK(iris::find_index_v<float,  type_list<int, float, double>> == 1);
+    STATIC_CHECK(iris::find_index_v<double, type_list<int, float, double>> == 2);
+    STATIC_CHECK(iris::find_index_v<int,    type_list<float, double>> == iris::find_npos);
+
+    STATIC_CHECK(iris::find_index_v<int, type_list<int, int, double>> == 0);
+}
diff --git a/test/type_traits.cpp b/test/type_traits.cpp
index 53c52c1..9f7fbc8 100644
--- a/test/type_traits.cpp
+++ b/test/type_traits.cpp
@@ -20,7 +20,7 @@ template<class... Ts>
 struct tuple;
 
 template<class... Ts>
-struct type_list;
+struct not_tuple;
 
 template<int... Ns>
 struct n_tuple;
@@ -618,9 +618,9 @@ TEST_CASE("specialization_of")
     STATIC_CHECK(iris::is_ttp_specialization_of_v<tuple<int>, tuple>);
     STATIC_CHECK(iris::is_ttp_specialization_of_v<tuple<int, double>, tuple>);
 
-    STATIC_CHECK(!iris::is_ttp_specialization_of_v<tuple<>, type_list>);
-    STATIC_CHECK(!iris::is_ttp_specialization_of_v<tuple<int>, type_list>);
-    STATIC_CHECK(!iris::is_ttp_specialization_of_v<tuple<int, double>, type_list>);
+    STATIC_CHECK(!iris::is_ttp_specialization_of_v<tuple<>, not_tuple>);
+    STATIC_CHECK(!iris::is_ttp_specialization_of_v<tuple<int>, not_tuple>);
+    STATIC_CHECK(!iris::is_ttp_specialization_of_v<tuple<int, double>, not_tuple>);
 
     STATIC_CHECK(iris::is_ctp_specialization_of_v<n_tuple<>, n_tuple>);
     STATIC_CHECK(iris::is_ctp_specialization_of_v<n_tuple<0>, n_tuple>);