[libc++][spaceship] Implement `operator<=>` for `forward_list`

[H-G-Hristov]

Summary

Implemented operator<=> for forward_list

[H-G-Hristov]

• Updated paper

[H-G-Hristov]

• Updated paper
• Added missing _LIBCPP_HIDE_FROM_ABI

[philnik]

LGTM!

[H-G-Hristov]

@philnik Thank you for commiting!

libcxx/docs/Status/SpaceshipProjects.csv

 | `[charconv.syn] <https://wg21.link/charconv.syn>`_,| `to_chars_result <https://reviews.llvm.org/D112366>`_,None,Mark de Wever,|Complete|
 | `[charconv.syn] <https://wg21.link/charconv.syn>`_,| `from_chars_result <https://reviews.llvm.org/D112366>`_,None,Mark de Wever,|Complete|
 | `[stacktrace.entry.cmp] <https://wg21.link/stacktrace.entry.cmp>`_,| stacktrace_entry,None,Unassigned,|Not Started|
 | `[stacktrace.basic.cmp] <https://wg21.link/stacktrace.basic.cmp>`_,| basic_stacktrace,[alg.three.way],Unassigned,|Not Started|
 | `[string.cmp] <https://wg21.link/string.cmp>`_,| `basic_string <https://reviews.llvm.org/D131421>`_,None,Mark de Wever,|Complete|
 | `[string.view.comparison] <https://wg21.link/string.view.comparison>`_,| `basic_string_view <https://reviews.llvm.org/D130295>`_,None,Mark de Wever,|Complete|
 | `[array.syn] <https://wg21.link/array.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),| `array <https://reviews.llvm.org/D132265>`_,[expos.only.func],Adrian Vogelsgesang,|In Progress|
 | `[deque.syn] <https://wg21.link/deque.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),| deque,[expos.only.func],Unassigned,|Not Started|
-| `[forward.list.syn] <https://wg21.link/forward.list.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),| forward_list,[expos.only.func],Unassigned,|Not Started|
+| `[forward.list.syn] <https://wg21.link/forward.list.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),| forward_list,[expos.only.func],Hristo Hristov,|Completed|
 | `[list.syn] <https://wg21.link/list.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),| `list <https://reviews.llvm.org/D132312>`_,[expos.only.func],Adrian Vogelsgesang,|Complete|
 | `[vector.syn] <https://wg21.link/vector.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),| `vector <https://reviews.llvm.org/D132268>`_,[expos.only.func],Adrian Vogelsgesang,|In Progress|
 | `[associative.map.syn] <https://wg21.link/associative.map.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),"| map
 | multimap",[expos.only.func],Unassigned,|Not Started|
 | `[associative.set.syn] <https://wg21.link/associative.set.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),"| multiset
 | set",[expos.only.func],Unassigned,|Not Started|
 | `[queue.ops] <https://wg21.link/queue.ops>`_,| queue,None,Unassigned,|Not Started|
 | `[stack.ops] <https://wg21.link/stack.ops>`_,| stack,None,Unassigned,|Not Started|

libcxx/include/forward_list

template <class InputIterator, class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
     -> forward_list<typename iterator_traits<InputIterator>::value_type, Allocator>;  // C++17
 
 template <class T, class Allocator>
     bool operator==(const forward_list<T, Allocator>& x,
                     const forward_list<T, Allocator>& y);
 
 template <class T, class Allocator>
     bool operator< (const forward_list<T, Allocator>& x,
-                    const forward_list<T, Allocator>& y);
+                    const forward_list<T, Allocator>& y); // removed in C++20
 
 template <class T, class Allocator>
     bool operator!=(const forward_list<T, Allocator>& x,
-                    const forward_list<T, Allocator>& y);
+                    const forward_list<T, Allocator>& y); // removed in C++20
 
 template <class T, class Allocator>
     bool operator> (const forward_list<T, Allocator>& x,
-                    const forward_list<T, Allocator>& y);
+                    const forward_list<T, Allocator>& y); // removed in C++20
 
 template <class T, class Allocator>
     bool operator>=(const forward_list<T, Allocator>& x,
-                    const forward_list<T, Allocator>& y);
+                    const forward_list<T, Allocator>& y); // removed in C++20
 
 template <class T, class Allocator>
     bool operator<=(const forward_list<T, Allocator>& x,
-                    const forward_list<T, Allocator>& y);
+                    const forward_list<T, Allocator>& y); // removed in C++20
+
+template<class T, class Allocator>
+    synth-three-way-result<T> operator<=>(const forward_list<T, Allocator>& x,
+                                          const forward_list<T, Allocator>& y); // since C++20
 
 template <class T, class Allocator>
     void swap(forward_list<T, Allocator>& x, forward_list<T, Allocator>& y)
          noexcept(noexcept(x.swap(y)));
 
 template <class T, class Allocator, class U>
     typename forward_list<T, Allocator>::size_type
     erase(forward_list<T, Allocator>& c, const U& value);       // C++20
 template <class T, class Allocator, class Predicate>
     typename forward_list<T, Allocator>::size_type
     erase_if(forward_list<T, Allocator>& c, Predicate pred);    // C++20
 
 }  // std
 
 */
 
 #include <__algorithm/comp.h>
 #include <__algorithm/lexicographical_compare.h>
+#include <__algorithm/lexicographical_compare_three_way.h>
 #include <__algorithm/min.h>
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
 #include <__iterator/distance.h>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/move_iterator.h>
 #include <__iterator/next.h>
 #include <__memory/addressof.h>
bool operator==(const forward_list<_Tp, _Alloc>& __x,
     _Ip __iy = __y.begin();
     _Ip __ey = __y.end();
     for (; __ix != __ex && __iy != __ey; ++__ix, ++__iy)
         if (!(*__ix == *__iy))
             return false;
     return (__ix == __ex) == (__iy == __ey);
 }
 
+#if _LIBCPP_STD_VER <= 17
+
 template <class _Tp, class _Alloc>
 inline _LIBCPP_INLINE_VISIBILITY
 bool operator!=(const forward_list<_Tp, _Alloc>& __x,
                 const forward_list<_Tp, _Alloc>& __y)
 {
     return !(__x == __y);
 }
 

 template <class _Tp, class _Alloc>
 inline _LIBCPP_INLINE_VISIBILITY
 bool operator<=(const forward_list<_Tp, _Alloc>& __x,
                 const forward_list<_Tp, _Alloc>& __y)
 {
     return !(__y < __x);
 }
 
+#else // #if _LIBCPP_STD_VER <= 17
+
+template<class _Tp, class _Allocator>
+__synth_three_way_result<_Tp> operator<=>(const forward_list<_Tp, _Allocator>& __x,
+                                          const forward_list<_Tp, _Allocator>& __y)
+{
+    return std::lexicographical_compare_three_way(
+        __x.begin(), __x.end(), __y.begin(), __y.end(), __synth_three_way);
+}
+
+#endif // #if _LIBCPP_STD_VER <= 17
+
 template <class _Tp, class _Alloc>
 inline _LIBCPP_INLINE_VISIBILITY
 void
 swap(forward_list<_Tp, _Alloc>& __x, forward_list<_Tp, _Alloc>& __y)
     _NOEXCEPT_(_NOEXCEPT_(__x.swap(__y)))
 {
     __x.swap(__y);
 }

libcxx/test/libcxx/containers/sequences/forwardlist/compare.three_way.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
+
+// <forward_list>
+
+// template<class T, class Allocator>
+//   synth-three-way-result<T> operator<=>(const forward_list<T, Allocator>& x,
+//                                         const forward_list<T, Allocator>& y);
+
+#include <cassert>
+#include <forward_list>
+
+#include "test_container_comparisons.h"
+
+int main(int, char**) {
+  assert(test_ordered_container_spaceship<std::forward_list>());
+  // `std::forward_list` is not constexpr, so no `static_assert` test here.
+  return 0;
+}