[libc++] Addresses LWG3755.

[Mordante]

Summary

LWG3755 tuple-for-each can call user-defined operator,

[philnik]

libcxx/test/libcxx/ranges/range.adaptors/range.adaptor.tuple/tuple-for-each.pass.cpp

2

IMO this should be tested through the public API. Otherwise this could regress easily if we refactor internals.

[ldionne]

libcxx/test/libcxx/ranges/range.adaptors/range.adaptor.tuple/tuple-for-each.pass.cpp

2

All the places that currently use tuple-for-each return void right now. I think that's the only way we have of testing this right now.

24

libcxx/docs/Status/Cxx2bIssues.csv

 "`3743 <https://wg21.link/LWG3743>`__","``ranges::to``'s reserve may be ill-formed", "November 2022","","","|ranges|"
 "`3745 <https://wg21.link/LWG3745>`__","``std::atomic_wait`` and its friends lack ``noexcept``", "November 2022","|Complete|","16.0",""
 "`3746 <https://wg21.link/LWG3746>`__","``optional``'s spaceship with ``U`` with a type derived from optional causes infinite constraint meta-recursion", "November 2022","","","|spaceship|"
 "`3747 <https://wg21.link/LWG3747>`__","``ranges::uninitialized_copy_n``, ``ranges::uninitialized_move_n``, and ``ranges::destroy_n`` should use ``std::move``", "November 2022","","","|ranges|"
 "`3750 <https://wg21.link/LWG3750>`__","Too many papers bump ``__cpp_lib_format``", "November 2022","","","|format|"
 "`3751 <https://wg21.link/LWG3751>`__","Missing feature macro for ``flat_set``", "November 2022","","","|flat_containers|"
 "`3753 <https://wg21.link/LWG3753>`__","Clarify entity vs. freestanding entity", "November 2022","","",""
 "`3754 <https://wg21.link/LWG3754>`__","Class template expected synopsis contains declarations that do not match the detailed description", "November 2022","|Nothing to do|","",""
-"`3755 <https://wg21.link/LWG3755>`__","``tuple-for-each`` can call ``user-defined`` ``operator,``", "November 2022","","",""
+"`3755 <https://wg21.link/LWG3755>`__","``tuple-for-each`` can call ``user-defined`` ``operator,``", "November 2022","|Complete|","17.0",""
 "`3757 <https://wg21.link/LWG3757>`__","What's the effect of ``std::forward_like<void>(x)``?", "November 2022","","",""
 "`3759 <https://wg21.link/LWG3759>`__","``ranges::rotate_copy`` should use ``std::move``", "November 2022","","","|ranges|"
 "`3760 <https://wg21.link/LWG3760>`__","``cartesian_product_view::iterator``'s ``parent_`` is never valid", "November 2022","","","|ranges|"
 "`3761 <https://wg21.link/LWG3761>`__","``cartesian_product_view::iterator::operator-`` should pass by reference", "November 2022","","","|ranges|"
 "`3762 <https://wg21.link/LWG3762>`__","``generator::iterator::operator==`` should pass by reference", "November 2022","","",""
 "`3764 <https://wg21.link/LWG3764>`__","``reference_wrapper::operator()`` should propagate noexcept", "November 2022","17.0","|Complete|",""
 "`3765 <https://wg21.link/LWG3765>`__","``const_sentinel`` should be constrained", "November 2022","","","|ranges|"
 "`3766 <https://wg21.link/LWG3766>`__","``view_interface::cbegin`` is underconstrained", "November 2022","","","|ranges|"

libcxx/include/__ranges/zip_view.h

return std::apply(
             std::invoke(__f, std::forward<_Types>(__elements))...);
       },
       std::forward<_Tuple>(__tuple));
 }
 
 template <class _Fun, class _Tuple>
 _LIBCPP_HIDE_FROM_ABI constexpr void __tuple_for_each(_Fun&& __f, _Tuple&& __tuple) {
   std::apply(
-      [&]<class... _Types>(_Types&&... __elements) { (std::invoke(__f, std::forward<_Types>(__elements)), ...); },
+      [&]<class... _Types>(_Types&&... __elements) {
+        (static_cast<void>(std::invoke(__f, std::forward<_Types>(__elements))), ...);
+      },
       std::forward<_Tuple>(__tuple));
 }
 
 template <class _Fun, class _Tuple1, class _Tuple2, size_t... _Indices>
 _LIBCPP_HIDE_FROM_ABI constexpr __tuple_or_pair<
     invoke_result_t<_Fun&, typename tuple_element<_Indices, remove_cvref_t<_Tuple1>>::type,
                     typename tuple_element<_Indices, remove_cvref_t<_Tuple2>>::type>...>
 __tuple_zip_transform(_Fun&& __f, _Tuple1&& __tuple1, _Tuple2&& __tuple2, index_sequence<_Indices...>) {

libcxx/test/libcxx/ranges/range.adaptors/range.adaptor.tuple/tuple-for-each.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <ranges>
+
+// template<class F, class Tuple>
+// constexpr void tuple-for-each(F&& f, Tuple&& t) { // exposition only
+
+// LWG3755 tuple-for-each can call user-defined operator,
+
+#include <ranges>
+#include <tuple>
+#include <cstdlib>
+
+struct Evil {
+  void operator,(Evil) { std::abort(); }
+};
+
+int main(int, char**) {
+  std::tuple<int, int> t;
+  std::ranges::__tuple_for_each([](int) { return Evil{}; }, t);
+
+  return 0;
+}