[libcxx-commits] [libcxx] [libc++] P2165R4: Update deduction guides for map containers and container adaptors (PR #136011)

[via libcxx-commits]

https://github.com/KSARK updated https://github.com/llvm/llvm-project/pull/136011

>From d6c3e29741d3ec176985f086ea7fffb7c59ccfe7 Mon Sep 17 00:00:00 2001
From: KSARK <hcwenyc at hotmail.com>
Date: Sat, 19 Apr 2025 16:25:42 +0000
Subject: [PATCH] [libc++] P2165R4: Update deduction guides for map containers
 and container adaptors

---
 libcxx/include/__iterator/iterator_traits.h   |  14 ++
 .../P2165R4_deduction_guides.pass.cpp         | 121 ++++++++++++++++++
 2 files changed, 135 insertions(+)
 create mode 100644 libcxx/test/std/containers/associative/associative.general/P2165R4_deduction_guides.pass.cpp

diff --git a/libcxx/include/__iterator/iterator_traits.h b/libcxx/include/__iterator/iterator_traits.h
index c08d72971ec3e..8e65f36c90c68 100644
--- a/libcxx/include/__iterator/iterator_traits.h
+++ b/libcxx/include/__iterator/iterator_traits.h
@@ -22,6 +22,7 @@
 #include <__fwd/pair.h>
 #include <__iterator/incrementable_traits.h>
 #include <__iterator/readable_traits.h>
+#include <__tuple/tuple_element.h>
 #include <__type_traits/common_reference.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/detected_or.h>
@@ -466,6 +467,18 @@ using __has_exactly_bidirectional_iterator_category _LIBCPP_NODEBUG =
 template <class _InputIterator>
 using __iter_value_type _LIBCPP_NODEBUG = typename iterator_traits<_InputIterator>::value_type;
 
+#if _LIBCPP_STD_VER >= 23
+template <class _InputIterator>
+using __iter_key_type _LIBCPP_NODEBUG = __remove_const_t<tuple_element_t<0, __iter_value_type<_InputIterator>>>;
+
+template <class _InputIterator>
+using __iter_mapped_type _LIBCPP_NODEBUG = tuple_element_t<1, __iter_value_type<_InputIterator>>;
+
+template <class _InputIterator>
+using __iter_to_alloc_type _LIBCPP_NODEBUG =
+    pair<const tuple_element_t<0, __iter_value_type<_InputIterator>>,
+         tuple_element_t<1, __iter_value_type<_InputIterator>>>;
+#else
 template <class _InputIterator>
 using __iter_key_type _LIBCPP_NODEBUG =
     __remove_const_t<typename iterator_traits<_InputIterator>::value_type::first_type>;
@@ -477,6 +490,7 @@ template <class _InputIterator>
 using __iter_to_alloc_type _LIBCPP_NODEBUG =
     pair<const typename iterator_traits<_InputIterator>::value_type::first_type,
          typename iterator_traits<_InputIterator>::value_type::second_type>;
+#endif // _LIBCPP_STD_VER >= 23
 
 template <class _Iter>
 using __iterator_category_type _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::iterator_category;
diff --git a/libcxx/test/std/containers/associative/associative.general/P2165R4_deduction_guides.pass.cpp b/libcxx/test/std/containers/associative/associative.general/P2165R4_deduction_guides.pass.cpp
new file mode 100644
index 0000000000000..8d55796f71c1e
--- /dev/null
+++ b/libcxx/test/std/containers/associative/associative.general/P2165R4_deduction_guides.pass.cpp
@@ -0,0 +1,121 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+#include <array>
+#include <flat_map>
+#include <iostream>
+#include <map>
+#include <set>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <unordered_map>
+#include <utility>
+#include <vector>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  // --- Input Data ---
+  // 1. Vector of std::pair
+  std::vector<std::pair<const int, std::string>> pair_vec = {{1, "apple"}, {2, "banana"}, {3, "cherry"}};
+
+  // 2. Vector of std::tuple
+  std::vector<std::tuple<int, double>> tuple_vec = {{10, 1.1}, {20, 2.2}, {30, 3.3}};
+
+  // 3. Vector of std::array
+  std::vector<std::array<long, 2>> array_vec = {{100L, 101L}, {200L, 201L}, {300L, 301L}};
+
+  // 4. Vector of std::pair with non-const key (for testing const addition in iter_to_alloc_type)
+  std::vector<std::pair<int, std::string>> non_const_key_pair_vec = {{5, "grape"}, {6, "kiwi"}};
+
+  // --- CTAD Tests ---
+
+  // map
+  std::map m1(pair_vec.begin(), pair_vec.end());
+  static_assert(std::is_same_v<decltype(m1), std::map<int, std::string>>);
+
+  // multimap
+  std::multimap mm1(pair_vec.begin(), pair_vec.end());
+  static_assert(std::is_same_v<decltype(mm1), std::multimap<int, std::string>>);
+
+  // unordered_map
+  std::unordered_map um1(pair_vec.begin(), pair_vec.end());
+  static_assert(std::is_same_v<decltype(um1), std::unordered_map<int, std::string>>);
+
+  // unordered_multimap
+  std::unordered_multimap umm1(pair_vec.begin(), pair_vec.end());
+  static_assert(std::is_same_v<decltype(umm1), std::unordered_multimap<int, std::string>>);
+
+  // flat_map
+  std::flat_map fm1(pair_vec.begin(), pair_vec.end());
+  static_assert(std::is_same_v<decltype(fm1), std::flat_map<int, std::string>>);
+
+  // flat_multimap
+  std::flat_multimap fmm1(pair_vec.begin(), pair_vec.end());
+  static_assert(std::is_same_v<decltype(fmm1), std::flat_multimap<int, std::string>>);
+
+  // map
+  std::map m2(tuple_vec.begin(), tuple_vec.end());
+  static_assert(std::is_same_v<decltype(m2), std::map<int, double>>);
+
+  // multimap
+  std::multimap mm2(tuple_vec.begin(), tuple_vec.end());
+  static_assert(std::is_same_v<decltype(mm2), std::multimap<int, double>>);
+
+  // unordered_map
+  std::unordered_map um2(tuple_vec.begin(), tuple_vec.end());
+  // Note: std::tuple needs a hash specialization to be used as a key in unordered containers.
+  // CTAD itself should work, but compilation/runtime might fail without a hash.
+  // This static_assert checks the deduced type. A hash specialization would be needed for actual use.
+  static_assert(std::is_same_v<decltype(um2), std::unordered_map<int, double>>);
+
+  // unordered_multimap
+  std::unordered_multimap umm2(tuple_vec.begin(), tuple_vec.end());
+  static_assert(std::is_same_v<decltype(umm2), std::unordered_multimap<int, double>>);
+
+  // flat_map
+  std::flat_map fm2(tuple_vec.begin(), tuple_vec.end());
+  static_assert(std::is_same_v<decltype(fm2), std::flat_map<int, double>>);
+
+  // flat_multimap
+  std::flat_multimap fmm2(tuple_vec.begin(), tuple_vec.end());
+  static_assert(std::is_same_v<decltype(fmm2), std::flat_multimap<int, double>>);
+
+  // map
+  std::map m3(array_vec.begin(), array_vec.end());
+  static_assert(std::is_same_v<decltype(m3), std::map<long, long>>);
+
+  // multimap
+  std::multimap mm3(array_vec.begin(), array_vec.end());
+  static_assert(std::is_same_v<decltype(mm3), std::multimap<long, long>>);
+
+  // unordered_map
+  std::unordered_map um3(array_vec.begin(), array_vec.end());
+  // Note: std::array needs a hash specialization.
+  static_assert(std::is_same_v<decltype(um3), std::unordered_map<long, long>>);
+
+  // unordered_multimap
+  std::unordered_multimap umm3(array_vec.begin(), array_vec.end());
+  static_assert(std::is_same_v<decltype(umm3), std::unordered_multimap<long, long>>);
+
+  // flat_map
+  std::flat_map fm3(array_vec.begin(), array_vec.end());
+  static_assert(std::is_same_v<decltype(fm3), std::flat_map<long, long>>);
+
+  // flat_multimap
+  std::flat_multimap fmm3(array_vec.begin(), array_vec.end());
+  static_assert(std::is_same_v<decltype(fmm3), std::flat_multimap<long, long>>);
+
+  // map
+  std::map m4(non_const_key_pair_vec.begin(), non_const_key_pair_vec.end());
+  static_assert(std::is_same_v<decltype(m4), std::map<int, std::string>>);
+
+  return 0;
+}