From faad3b704f3433b207adfba438a16ee5257f8a5c Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Tue, 20 Oct 2026 22:32:02 -0600
Subject: [PATCH] :bug: Fix filtering indexed tuples

Problem:
- Filtering an `indexed_tuple` doesn't work.

Solution:
- Fix it: keep the indices while filtering the elements.
---
 include/stdx/tuple_algorithms.hpp | 16 ++++++++++++++--
 test/tuple_algorithms.cpp         |  9 +++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/include/stdx/tuple_algorithms.hpp b/include/stdx/tuple_algorithms.hpp
index be86d1f..011b36c 100644
--- a/include/stdx/tuple_algorithms.hpp
+++ b/include/stdx/tuple_algorithms.hpp
@@ -133,6 +133,18 @@ template <has_tuple_protocol Tup, typename T>
     return tuple_snoc(std::forward<Tup>(tup), std::forward<T>(t));
 }
 
+namespace detail {
+template <typename T> struct tuple_variation {
+    template <typename... Us>
+    using fn = boost::mp11::mp_assign<T, type_list<Us...>>;
+};
+
+template <typename Idx, typename... Ts>
+struct tuple_variation<indexed_tuple<Idx, Ts...>> {
+    template <typename... Us> using fn = indexed_tuple<Idx, Us...>;
+};
+} // namespace detail
+
 template <template <typename T> typename Pred, has_tuple_protocol T>
 [[nodiscard]] constexpr auto filter(T &&t) {
     using tuple_t = std::remove_cvref_t<T>;
@@ -157,8 +169,8 @@ template <template <typename T> typename Pred, has_tuple_protocol T>
         }();
 
         return [&]<std::size_t... Js>(std::index_sequence<Js...>) {
-            using R = boost::mp11::mp_assign<
-                tuple_t,
+            using R = boost::mp11::mp_apply_q<
+                detail::tuple_variation<tuple_t>,
                 type_list<stdx::tuple_element_t<indices[Js], tuple_t>...>>;
             return R{get<indices[Js]>(std::forward<T>(t))...};
         }(std::make_index_sequence<num_matches>{});
diff --git a/test/tuple_algorithms.cpp b/test/tuple_algorithms.cpp
index d0dc7c4..232c690 100644
--- a/test/tuple_algorithms.cpp
+++ b/test/tuple_algorithms.cpp
@@ -543,6 +543,15 @@ TEST_CASE("filter", "[tuple_algorithms]") {
                                     std::integral_constant<int, 4>{}});
 }
 
+TEST_CASE("filter an indexed_tuple", "[tuple_algorithms]") {
+    constexpr auto t = stdx::indexed_tuple{
+        std::integral_constant<int, 1>{}, std::integral_constant<int, 2>{},
+        std::integral_constant<int, 3>{}, std::integral_constant<int, 4>{}};
+    constexpr auto u = stdx::filter<is_even>(t);
+    STATIC_REQUIRE(u == stdx::indexed_tuple{std::integral_constant<int, 2>{},
+                                            std::integral_constant<int, 4>{}});
+}
+
 TEST_CASE("filter on std::tuple", "[tuple_algorithms]") {
     constexpr auto t = std::tuple{
         std::integral_constant<int, 1>{}, std::integral_constant<int, 2>{},