[libc++][variant] P2637R3: Member visit (std::variant) (#76447)

Implements parts of: `P2637R3` https://wg21.link/P2637R3
(https://eel.is/c++draft/variant.visit)

Implements:
`variant.visit()`
`variant.visit<R>()`

The tests are as close as possible to the non-member function.

To land after: #76268

---------

Co-authored-by: Zingam <zingam@outlook.com>

Author: H-G-Hristov

libcxx/docs/Status/Cxx2cPapers.csv

@@ -17,7 +17,7 @@
 "`P0792R14 <https://wg21.link/P0792R14>`__","LWG","``function_ref``: a type-erased callable reference","Varna June 2023","","",""
 "`P2874R2 <https://wg21.link/P2874R2>`__","LWG","Mandating Annex D Require No More","Varna June 2023","","",""
 "`P2757R3 <https://wg21.link/P2757R3>`__","LWG","Type-checking format args","Varna June 2023","","","|format|"
-"`P2637R3 <https://wg21.link/P2637R3>`__","LWG","Member ``visit``","Varna June 2023","","","|format|"
+"`P2637R3 <https://wg21.link/P2637R3>`__","LWG","Member ``visit``","Varna June 2023","|Partial|","18.0",""
 "`P2641R4 <https://wg21.link/P2641R4>`__","CWG, LWG","Checking if a ``union`` alternative is active","Varna June 2023","","",""
 "`P1759R6 <https://wg21.link/P1759R6>`__","LWG","Native handles and file streams","Varna June 2023","|Complete|","18.0",""
 "`P2697R1 <https://wg21.link/P2697R1>`__","LWG","Interfacing ``bitset`` with ``string_view``","Varna June 2023","|Complete|","18.0",""

libcxx/include/__config

@@ -1513,6 +1513,11 @@ __sanitizer_verify_double_ended_contiguous_container(const void*, const void*, c
 #    define _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 #  endif
 
+// Clang-18 has support for deducing this, but it does not set the FTM.
+#  if defined(__cpp_explicit_this_parameter) || (defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER >= 1800)
+#    define _LIBCPP_HAS_EXPLICIT_THIS_PARAMETER
+#  endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP___CONFIG

libcxx/include/variant

@@ -69,6 +69,12 @@ namespace std {
 
     // 20.7.2.6, swap
     void swap(variant&) noexcept(see below);
+
+    // [variant.visit], visitation
+    template<class Self, class Visitor>
+      constexpr decltype(auto) visit(this Self&&, Visitor&&); // Since C++26
+    template<class R, class Self, class Visitor>
+      constexpr R visit(this Self&&, Visitor&&);              // Since C++26
   };
 
   // 20.7.3, variant helper classes
@@ -235,6 +241,7 @@ namespace std {
 #include <__type_traits/void_t.h>
 #include <__utility/declval.h>
 #include <__utility/forward.h>
+#include <__utility/forward_like.h>
 #include <__utility/in_place.h>
 #include <__utility/move.h>
 #include <__utility/swap.h>
@@ -1130,6 +1137,19 @@ using __best_match_t = typename invoke_result_t<_MakeOverloads<_Types...>, _Tp,
 
 } // namespace __variant_detail
 
+template <class _Visitor, class... _Vs, typename = void_t<decltype(std::__as_variant(std::declval<_Vs>()))...>>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr decltype(auto)
+visit(_Visitor&& __visitor, _Vs&&... __vs);
+
+#  if _LIBCPP_STD_VER >= 20
+template <class _Rp,
+          class _Visitor,
+          class... _Vs,
+          typename = void_t<decltype(std::__as_variant(std::declval<_Vs>()))...>>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr _Rp
+visit(_Visitor&& __visitor, _Vs&&... __vs);
+#  endif
+
 template <class... _Types>
 class _LIBCPP_TEMPLATE_VIS _LIBCPP_DECLSPEC_EMPTY_BASES variant
     : private __sfinae_ctor_base< __all<is_copy_constructible_v<_Types>...>::value,
@@ -1273,6 +1293,27 @@ public:
     __impl_.__swap(__that.__impl_);
   }
 
+#  if _LIBCPP_STD_VER >= 26 && defined(_LIBCPP_HAS_EXPLICIT_THIS_PARAMETER)
+  // Helper class to implement [variant.visit]/10
+  //   Constraints: The call to visit does not use an explicit template-argument-list
+  //   that begins with a type template-argument.
+  struct __variant_visit_barrier_tag {
+    _LIBCPP_HIDE_FROM_ABI explicit __variant_visit_barrier_tag() = default;
+  };
+
+  template <__variant_visit_barrier_tag = __variant_visit_barrier_tag{}, class _Self, class _Visitor>
+  _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) visit(this _Self&& __self, _Visitor&& __visitor) {
+    using _VariantT = _OverrideRef<_Self&&, _CopyConst<remove_reference_t<_Self>, variant>>;
+    return std::visit(std::forward<_Visitor>(__visitor), (_VariantT)__self);
+  }
+
+  template <class _Rp, class _Self, class _Visitor>
+  _LIBCPP_HIDE_FROM_ABI constexpr _Rp visit(this _Self&& __self, _Visitor&& __visitor) {
+    using _VariantT = _OverrideRef<_Self&&, _CopyConst<remove_reference_t<_Self>, variant>>;
+    return std::visit<_Rp>(std::forward<_Visitor>(__visitor), (_VariantT)__self);
+  }
+#  endif
+
 private:
   __variant_detail::__impl<_Types...> __impl_;
 
@@ -1511,7 +1552,7 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr vo
   }
 }
 
-template < class _Visitor, class... _Vs, typename = void_t<decltype(std::__as_variant(std::declval<_Vs>()))...> >
+template < class _Visitor, class... _Vs, typename>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr decltype(auto)
 visit(_Visitor&& __visitor, _Vs&&... __vs) {
   using __variant_detail::__visitation::__variant;
@@ -1520,10 +1561,7 @@ visit(_Visitor&& __visitor, _Vs&&... __vs) {
 }
 
 #  if _LIBCPP_STD_VER >= 20
-template < class _Rp,
-           class _Visitor,
-           class... _Vs,
-           typename = void_t<decltype(std::__as_variant(std::declval<_Vs>()))...> >
+template < class _Rp, class _Visitor, class... _Vs, typename>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr _Rp
 visit(_Visitor&& __visitor, _Vs&&... __vs) {
   using __variant_detail::__visitation::__variant;

libcxx/test/std/utilities/variant/variant.visit.member/robust_against_adl.pass.cpp

@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++23
+// The tested functionality needs deducing this.
+// UNSUPPORTED: clang-16 || clang-17
+// XFAIL: apple-clang
+
+// <variant>
+
+// class variant;
+// template<class Self, class Visitor>
+//   constexpr decltype(auto) visit(this Self&&, Visitor&&); // since C++26
+// template<class R, class Self, class Visitor>
+//   constexpr R visit(this Self&&, Visitor&&);              // since C++26
+
+#include <variant>
+
+#include "test_macros.h"
+
+struct Incomplete;
+template <class T>
+struct Holder {
+  T t;
+};
+
+constexpr bool test(bool do_it) {
+  if (do_it) {
+    std::variant<Holder<Incomplete>*, int> v = nullptr;
+
+    v.visit([](auto) {});
+    v.visit([](auto) -> Holder<Incomplete>* { return nullptr; });
+    v.visit<void>([](auto) {});
+    v.visit<void*>([](auto) -> Holder<Incomplete>* { return nullptr; });
+  }
+  return true;
+}
+
+int main(int, char**) {
+  test(true);
+  static_assert(test(true));
+
+  return 0;
+}