[libcxx-commits] [libcxx] [libc++] Implement comparison operators for `tuple` added in C++23 (PR #148799)

[via libcxx-commits]

================
@@ -288,6 +291,72 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #  ifndef _LIBCPP_CXX03_LANG
 
+template <size_t _Ip>
+struct __tuple_equal {
+  template <class _Tp, class _Up>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool operator()(const _Tp& __x, const _Up& __y) {
+    return __tuple_equal<_Ip - 1>()(__x, __y) && std::get<_Ip - 1>(__x) == std::get<_Ip - 1>(__y);
+  }
+
+#    if _LIBCPP_STD_VER >= 26
+  template <class _Tp, class _Up>
+  static constexpr bool __can_compare =
+      __tuple_equal<_Ip - 1>::template __can_compare<_Tp, _Up> && requires(const _Tp& __x, const _Up& __y) {
+        { std::get<_Ip - 1>(__x) == std::get<_Ip - 1>(__y) } -> __boolean_testable;
+      };
+#    endif // _LIBCPP_STD_VER >= 26
+};
+
+template <>
+struct __tuple_equal<0> {
+  template <class _Tp, class _Up>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool operator()(const _Tp&, const _Up&) {
+    return true;
+  }
+
+#    if _LIBCPP_STD_VER >= 26
+  template <class _Tp, class _Up>
+  static constexpr bool __can_compare = true;
+#    endif // _LIBCPP_STD_VER >= 26
+};
+
+#    if _LIBCPP_STD_VER >= 20
+template <class _Ret, class _Tp, class _Up, size_t... _Is>
+_LIBCPP_HIDE_FROM_ABI constexpr _Ret __tuple_compare_three_way(const _Tp& __x, const _Up& __y, index_sequence<_Is...>) {
+  _Ret __result = strong_ordering::equal;
+  static_cast<void>(
+      ((__result = std::__synth_three_way(std::get<_Is>(__x), std::get<_Is>(__y)), __result != 0) || ...));
+  return __result;
+}
+#    endif // _LIBCPP_STD_VER >= 20
+
+#    if _LIBCPP_STD_VER >= 23
+template <class>
+inline constexpr bool __is_tuple_v = false;
+
+template <class... _Tp>
+inline constexpr bool __is_tuple_v<tuple<_Tp...>> = true;
+
+template <class _Tp>
+concept __tuple_like_no_tuple = __tuple_like<_Tp> && !__is_tuple_v<_Tp>;
+
+template <class _Tp, class _Up, class _IndexSeq>
+struct __tuple_common_comparison_category_impl {};
+template <class _Tp, class _Up, size_t... _Indices>
+  requires(tuple_size_v<_Tp> == tuple_size_v<_Up>) && requires {
----------------
h-vetinari wrote:

This could be a `FIXME:` comment?

https://github.com/llvm/llvm-project/pull/148799