[libc++][spaceship] Implement operator<=> for deque

Based on https://reviews.llvm.org/D132312
Dependes on https://reviews.llvm.org/D132312

Reviewed By: #libc, Mordante, philnik

Spies: philnik, Mordante, yaxunl, libcxx-commits

Differential Revision: https://reviews.llvm.org/D144821

libcxx/docs/Status/SpaceshipProjects.csv

@@ -36,7 +36,7 @@ Section,Description,Dependencies,Assignee,Complete
 | `[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|
+| `[deque.syn] <https://wg21.link/deque.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),| deque <https://reviews.llvm.org/D144821>,[expos.only.func],Hristo Hristov,|Complete|
 | `[forward.list.syn] <https://wg21.link/forward.list.syn>`_ (`general <https://wg21.link/container.opt.reqmts>`_),| forward_list,[expos.only.func],Unassigned,|Not Started|
 | `[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|

libcxx/include/deque

@@ -134,15 +134,18 @@ template <class InputIterator, class Allocator = allocator<typename iterator_tra
 template <class T, class Allocator>
     bool operator==(const deque<T,Allocator>& x, const deque<T,Allocator>& y);
 template <class T, class Allocator>
-    bool operator< (const deque<T,Allocator>& x, const deque<T,Allocator>& y);
+    bool operator< (const deque<T,Allocator>& x, const deque<T,Allocator>& y); // removed in C++20
 template <class T, class Allocator>
-    bool operator!=(const deque<T,Allocator>& x, const deque<T,Allocator>& y);
+    bool operator!=(const deque<T,Allocator>& x, const deque<T,Allocator>& y); // removed in C++20
 template <class T, class Allocator>
-    bool operator> (const deque<T,Allocator>& x, const deque<T,Allocator>& y);
+    bool operator> (const deque<T,Allocator>& x, const deque<T,Allocator>& y); // removed in C++20
 template <class T, class Allocator>
-    bool operator>=(const deque<T,Allocator>& x, const deque<T,Allocator>& y);
+    bool operator>=(const deque<T,Allocator>& x, const deque<T,Allocator>& y); // removed in C++20
 template <class T, class Allocator>
-    bool operator<=(const deque<T,Allocator>& x, const deque<T,Allocator>& y);
+    bool operator<=(const deque<T,Allocator>& x, const deque<T,Allocator>& y); // removed in C++20
+template<class T, class Allocator>
+    synth-three-way-result<T> operator<=>(const deque<T, Allocator>& x,
+                                          const deque<T, Allocator>& y);       // since C++20
 
 // specialized algorithms:
 template <class T, class Allocator>
@@ -165,6 +168,7 @@ template <class T, class Allocator, class Predicate>
 #include <__algorithm/equal.h>
 #include <__algorithm/fill_n.h>
 #include <__algorithm/lexicographical_compare.h>
+#include <__algorithm/lexicographical_compare_three_way.h>
 #include <__algorithm/min.h>
 #include <__algorithm/remove.h>
 #include <__algorithm/remove_if.h>
@@ -2342,6 +2346,8 @@ operator==(const deque<_Tp, _Allocator>& __x, const deque<_Tp, _Allocator>& __y)
     return __sz == __y.size() && _VSTD::equal(__x.begin(), __x.end(), __y.begin());
 }
 
+#if _LIBCPP_STD_VER <= 17
+
 template <class _Tp, class _Allocator>
 inline _LIBCPP_HIDE_FROM_ABI
 bool
@@ -2382,6 +2388,19 @@ operator<=(const deque<_Tp, _Allocator>& __x, const deque<_Tp, _Allocator>& __y)
     return !(__y < __x);
 }
 
+#else // _LIBCPP_STD_VER <= 17
+
+template<class _Tp, class _Allocator>
+inline _LIBCPP_HIDE_FROM_ABI
+__synth_three_way_result<_Tp>
+operator<=>(const deque<_Tp, _Allocator>& __x, const deque<_Tp, _Allocator>& __y)
+{
+    return std::lexicographical_compare_three_way(
+        __x.begin(), __x.end(), __y.begin(), __y.end(), __synth_three_way);
+}
+
+#endif // _LIBCPP_STD_VER <= 17
+
 template <class _Tp, class _Allocator>
 inline _LIBCPP_HIDE_FROM_ABI
 void

libcxx/test/libcxx/containers/sequences/deque/compare.three_way.pass.cpp

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <deque>
+
+// template<class T, class Allocator>
+//   synth-three-way-result<T> operator<=>(const deque<T, Allocator>& x,
+//                                         const deque<T, Allocator>& y);
+
+#include <cassert>
+#include <deque>
+
+#include "test_container_comparisons.h"
+
+int main(int, char**) {
+  assert(test_ordered_container_spaceship<std::deque>());
+  // `std::deque` is not constexpr, so no `static_assert` test here.
+  return 0;
+}