✨ Add tuple_cons and tuple_snoc

docs/tuple_algorithms.adoc

@@ -22,6 +22,10 @@ contains various (free function) algorithms that work on `stdx::tuple`.
 * `to_unordered_set` - produce a tuple of unique types that are in the order given
 * `transform` - a variadic transform on tuple(s)
 * `tuple_cat` - like `std::tuple_cat`
+* `tuple_cons` - add an element to the front of a tuple
+* `tuple_push_back` - alias for `tuple_snoc`
+* `tuple_push_front` - alias for `tuple_cons`
+* `tuple_snoc` - add an element to the back of a tuple
 * `unique` - produce a tuple where adjacent types that are the same are merged into one element (the first such)
 
 === `all_of`, `any_of`, `none_of`
@@ -278,6 +282,32 @@ auto x = get<X>(t).value; // 42
 `tuple_cat` works just like
 https://en.cppreference.com/w/cpp/utility/tuple/tuple_cat[`std::tuple_cat`].
 
+=== `tuple_cons`/`tuple_push_front`
+
+`tuple_cons` adds an item to the front of a tuple. `tuple_push_front` is an
+alias for `tuple_cons`.
+
+[source,cpp]
+----
+auto t = stdx::tuple_cons(1, stdx:tuple{2, 3}); // {1, 2, 3}
+----
+
+NOTE: `tuple_cons` preserves the reference qualifiers in the given tuple, but
+decays the "single" argument, as `make_tuple` does.
+
+=== `tuple_snoc`/`tuple_push_back`
+
+`tuple_snoc` adds an item to the back of a tuple. `tuple_push_back` is an alias
+for `tuple_snoc`.
+
+[source,cpp]
+----
+auto t = stdx::tuple_snoc(stdx:tuple{2, 3}, 1); // {2, 3, 1}
+----
+
+NOTE: `tuple_snoc` preserves the reference qualifiers in the given tuple, but
+decays the "single" argument, as `make_tuple` does.
+
 === `unique`
 
 `unique` works like

include/stdx/tuple_algorithms.hpp

@@ -49,6 +49,38 @@ template <tuplelike... Ts> [[nodiscard]] constexpr auto tuple_cat(Ts &&...ts) {
     }
 }
 
+template <typename T, tuplelike Tup>
+[[nodiscard]] constexpr auto tuple_cons(T &&t, Tup &&tup) {
+    using tuple_t = std::remove_cvref_t<Tup>;
+    return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
+        return stdx::tuple<std::remove_cvref_t<T>,
+                           stdx::tuple_element_t<Is, tuple_t>...>{
+            std::forward<T>(t), std::forward<Tup>(tup)[index<Is>]...};
+    }(std::make_index_sequence<stdx::tuple_size_v<tuple_t>>{});
+}
+
+template <tuplelike Tup, typename T>
+[[nodiscard]] constexpr auto tuple_snoc(Tup &&tup, T &&t) {
+    using tuple_t = std::remove_cvref_t<Tup>;
+    return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
+        return stdx::tuple<stdx::tuple_element_t<Is, tuple_t>...,
+                           std::remove_cvref_t<T>>{
+            std::forward<Tup>(tup)[index<Is>]..., std::forward<T>(t)};
+    }(std::make_index_sequence<stdx::tuple_size_v<tuple_t>>{});
+}
+
+template <typename T, tuplelike Tup>
+[[nodiscard]] constexpr auto tuple_push_front(T &&t,
+                                              Tup &&tup) -> decltype(auto) {
+    return tuple_cons(std::forward<T>(t), std::forward<Tup>(tup));
+}
+
+template <tuplelike Tup, typename T>
+[[nodiscard]] constexpr auto tuple_push_back(Tup &&tup,
+                                             T &&t) -> decltype(auto) {
+    return tuple_snoc(std::forward<Tup>(tup), std::forward<T>(t));
+}
+
 template <template <typename T> typename Pred, tuplelike T>
 [[nodiscard]] constexpr auto filter(T &&t) {
     using tuple_t = std::remove_cvref_t<T>;

test/tuple_algorithms.cpp

@@ -722,3 +722,49 @@ TEST_CASE("gather_by with projection", "[tuple_algorithms]") {
               0, [](auto x, auto y) { return x + y.value; }) == 23);
     CHECK(get<2>(gathered) == stdx::tuple{named_int<C>{3}});
 }
+
+TEST_CASE("tuple_cons", "[tuple_algorithms]") {
+    static_assert(stdx::tuple_cons(1, stdx::tuple{}) == stdx::tuple{1});
+    auto t = stdx::tuple_cons(1, stdx::tuple{});
+    static_assert(std::is_same_v<decltype(t), stdx::tuple<int>>);
+}
+
+TEST_CASE("tuple_cons (move only)", "[tuple_algorithms]") {
+    auto t = stdx::tuple_cons(move_only{5}, stdx::tuple{move_only{10}});
+    static_assert(
+        std::is_same_v<decltype(t), stdx::tuple<move_only, move_only>>);
+    CHECK(t == stdx::tuple{move_only{5}, move_only{10}});
+}
+
+TEST_CASE("tuple_cons (references)", "[tuple_algorithms]") {
+    auto x = 1;
+    auto t = stdx::tuple_cons(1, stdx::tuple<int &>{x});
+    static_assert(std::is_same_v<decltype(t), stdx::tuple<int, int &>>);
+}
+
+TEST_CASE("tuple_snoc", "[tuple_algorithms]") {
+    static_assert(stdx::tuple_snoc(stdx::tuple{}, 1) == stdx::tuple{1});
+    auto t = stdx::tuple_snoc(stdx::tuple{}, 1);
+    static_assert(std::is_same_v<decltype(t), stdx::tuple<int>>);
+}
+
+TEST_CASE("tuple_snoc (move only)", "[tuple_algorithms]") {
+    auto t = stdx::tuple_snoc(stdx::tuple{move_only{10}}, move_only{5});
+    static_assert(
+        std::is_same_v<decltype(t), stdx::tuple<move_only, move_only>>);
+    CHECK(t == stdx::tuple{move_only{10}, move_only{5}});
+}
+
+TEST_CASE("tuple_snoc (references)", "[tuple_algorithms]") {
+    auto x = 1;
+    auto t = stdx::tuple_snoc(stdx::tuple<int &>{x}, 1);
+    static_assert(std::is_same_v<decltype(t), stdx::tuple<int &, int>>);
+}
+
+TEST_CASE("tuple_push_front", "[tuple_algorithms]") {
+    static_assert(stdx::tuple_push_front(1, stdx::tuple{}) == stdx::tuple{1});
+}
+
+TEST_CASE("tuple_push_back", "[tuple_algorithms]") {
+    static_assert(stdx::tuple_push_back(stdx::tuple{}, 1) == stdx::tuple{1});
+}