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

[H-G-Hristov]

Summary

Depends on D146066

Depends on D132268

Implements parts of P1614R2 operator<=> for stack

[H-G-Hristov]

• Added URL

[H-G-Hristov]

Fix CI

[H-G-Hristov]

• Fix CI

[H-G-Hristov]

Rebased

[H-G-Hristov]

Rebased

[H-G-Hristov]

Addressed review comments

[H-G-Hristov]

Rebased

[H-G-Hristov]

Rebased + Fix CI

[H-G-Hristov]

Rebased

[Mordante]

LGTM after addressing the review comments.

libcxx/include/stack

106

I miss an update of SpaceshipProjects.csv

411

If possible we prefer to do exactly what the Standard says. This is consistent with operator== and operator<.

[H-G-Hristov]

@Mordante Thank you for the review.

libcxx/include/stack

411

c is a protected member of stack, which requires to declare the operator<=> as a friend similar to operator== and operator<. Such implementation compiles fine with Apple Clang 14.0.3 (LLVM 15) but it fails to compile on the CI with newer Clang 16/17 (I think) with myriad of ambiguous operator declarations errors all over the place. I did some research but all of this was over my head, so I settled on the above implementation.
I did the same for queue. I'll refrain to land these patches until I get more feedback.

[Mordante]

libcxx/include/stack

411

I really would like to know what the issue with friends are. I tested with queue and it works for me. I will post more info on that patch.

[H-G-Hristov]

Rebased + addressed comments

[H-G-Hristov]

@Mordante Thank you for looking into this. I left a comment according to https://reviews.llvm.org/D146066 and I marked the comments as Done.
I'll rebase the patch later to get the CI green again.

[H-G-Hristov]

Rebased

[H-G-Hristov]

Rebased

[H-G-Hristov]

Rebased

[H-G-Hristov]

Rebased

libcxx/docs/Status/SpaceshipProjects.csv

 | `[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
 | Hristo Hristov",|Complete|
 | `[associative.map.syn] <https://wg21.link/associative.map.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),"| `map <https://reviews.llvm.org/D145976>`_
 | `multimap <https://reviews.llvm.org/D145976>`_",[expos.only.func],Hristo Hristov,|Complete|
 | `[associative.set.syn] <https://wg21.link/associative.set.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),"| multiset
 | set",[expos.only.func],Hristo Hristov,|In Progress|
 | `[queue.ops] <https://wg21.link/queue.ops>`_,| `queue <https://reviews.llvm.org/D146066>`_,None,Hristo Hristov,|Complete|
-| `[stack.ops] <https://wg21.link/stack.ops>`_,| `stack <https://reviews.llvm.org/D146094>`_,None,Hristo Hristov,|In Progress|
+| `[stack.ops] <https://wg21.link/stack.ops>`_,| `stack <https://reviews.llvm.org/D146094>`_,None,Hristo Hristov,|Complete|
 | `[reverse.iter.cmp] <https://wg21.link/reverse.iter.cmp>`_,| `reverse_iterator <https://reviews.llvm.org/D113695>`_,None,Mikhail Maltsev,|Complete|
 | `[move.iter.op.comp] <https://wg21.link/move.iter.op.comp>`_,| `move_iterator <https://reviews.llvm.org/D117656>`_,None,Arthur O'Dwyer,|Complete|
 | `[counted.iter.cmp] <https://wg21.link/counted.iter.cmp>`_,| `counted_iterator <https://reviews.llvm.org/D106205>`_,None,Zoe Carver,|Complete|
 | `[range.iota.iterator] <https://wg21.link/range.iota.iterator>`_,| `ranges::iota_view::iterator <https://reviews.llvm.org/D110774>`_,[concepts.cmp],Arthur O'Dwyer,|Complete|
 | `[range.transform.iterator] <https://wg21.link/range.transform.iterator>`_,| `ranges::transform_view::iterator <https://reviews.llvm.org/D110774>`_,[concepts.cmp],Arthur O'Dwyer,|Complete|
 | `[range.elements.iterator] <https://wg21.link/range.elements.iterator>`_,| ranges::elements_view::iterator,[concepts.cmp],Hui Xie,|Complete|
 | `[time.duration.comparisons] <https://wg21.link/time.duration.comparisons>`_, `chrono::duration <https://reviews.llvm.org/D145881>`_, None, Hristo Hristov, |Complete|
 | `[time.point.comparisons] <https://wg21.link/time.point.comparisons>`_, `chrono::time_point <https://reviews.llvm.org/D146250>`_, None, Hristo Hristov, |Complete|

libcxx/include/stack

 template <class T, class Container>
   bool operator!=(const stack<T, Container>& x, const stack<T, Container>& y);
 template <class T, class Container>
   bool operator> (const stack<T, Container>& x, const stack<T, Container>& y);
 template <class T, class Container>
   bool operator>=(const stack<T, Container>& x, const stack<T, Container>& y);
 template <class T, class Container>
   bool operator<=(const stack<T, Container>& x, const stack<T, Container>& y);
+template<class T, three_way_comparable Container>
+  compare_three_way_result_t<Container>
+    operator<=>(const stack<T, Container>& x, const stack<T, Container>& y); // since C++20
 
 template <class T, class Container>
   void swap(stack<T, Container>& x, stack<T, Container>& y)
   noexcept(noexcept(x.swap(y)));
 
 }  // std
 
 */

 template <class _Tp, class _Container>
 inline _LIBCPP_INLINE_VISIBILITY
 bool
 operator<=(const stack<_Tp, _Container>& __x, const stack<_Tp, _Container>& __y)
 {
     return !(__y < __x);
 }
 
+#if _LIBCPP_STD_VER >= 20
+
+template <class _Tp, three_way_comparable _Container>
+_LIBCPP_HIDE_FROM_ABI compare_three_way_result_t<_Container>
+operator<=>(const stack<_Tp, _Container>& __x, const stack<_Tp, _Container>& __y) {
+    // clang 16 bug: declaring `friend operator<=>` causes "use of overloaded operator '*' is ambiguous" errors
+    return __x.__get_container() <=> __y.__get_container();
+}
+
+#endif
+
 template <class _Tp, class _Container>
 inline _LIBCPP_INLINE_VISIBILITY
 __enable_if_t<__is_swappable<_Container>::value, void>
 swap(stack<_Tp, _Container>& __x, stack<_Tp, _Container>& __y)
     _NOEXCEPT_(_NOEXCEPT_(__x.swap(__y)))
 {
     __x.swap(__y);
 }

libcxx/test/std/containers/container.adaptors/stack/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
+
+// <stack>
+
+// template<class T, three_way_comparable Container>
+//   compare_three_way_result_t<Container>
+//     operator<=>(const stack<T, Container>& x, const stack<T, Container>& y);
+
+#include <cassert>
+#include <deque>
+#include <list>
+#include <stack>
+#include <vector>
+
+#include "nasty_containers.h"
+#include "test_container_comparisons.h"
+
+int main(int, char**) {
+  assert((test_sequence_container_adaptor_spaceship<std::stack, std::deque>()));
+  assert((test_sequence_container_adaptor_spaceship<std::stack, std::list>()));
+  assert((test_sequence_container_adaptor_spaceship<std::stack, std::vector>()));
+  assert((test_sequence_container_adaptor_spaceship<std::stack, nasty_list>()));
+  assert((test_sequence_container_adaptor_spaceship<std::stack, nasty_vector>()));
+  // `std::stack` is not constexpr, so no `static_assert` test here.
+  return 0;
+}