[libcxx-commits] [libcxx] [libc++] LWG4497: `std::nullopt_t` should be comparable (PR #195549)

[Hristo Hristov via libcxx-commits]

================
@@ -362,6 +362,16 @@ struct nullopt_t {
     explicit __secret_tag() = default;
   };
   _LIBCPP_HIDE_FROM_ABI constexpr explicit nullopt_t(__secret_tag, __secret_tag) noexcept {}
+#    if _LIBCPP_STD_VER <= 17
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator==(nullopt_t, nullopt_t) { return true; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator!=(nullopt_t, nullopt_t) { return false; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator>(nullopt_t, nullopt_t) { return false; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator<(nullopt_t, nullopt_t) { return false; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator>=(nullopt_t, nullopt_t) { return true; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator<=(nullopt_t, nullopt_t) { return true; }
+#    else
+  _LIBCPP_HIDE_FROM_ABI constexpr friend strong_ordering operator<=>(nullopt_t, nullopt_t) = default;
+#    endif
----------------
Zingam wrote:

```suggestion
_LIBCPP_HIDE_FROM_ABI constexpr explicit nullopt_t(__secret_tag, __secret_tag) noexcept {}

#    if _LIBCPP_STD_VER <= 17
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator==(nullopt_t, nullopt_t) noexcept { return true; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator!=(nullopt_t, nullopt_t) noexcept { return false; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator>(nullopt_t, nullopt_t) noexcept { return false; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator<(nullopt_t, nullopt_t) noexcept { return false; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator>=(nullopt_t, nullopt_t) noexcept { return true; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator<=(nullopt_t, nullopt_t) noexcept { return true; }
#    else
  _LIBCPP_HIDE_FROM_ABI constexpr friend strong_ordering operator<=>(nullopt_t, nullopt_t) noexcept = default;
#    endif
```

IMO Similar to https://github.com/llvm/llvm-project/blob/305469117a2daad5582d7d90f0f4f029ea3aa3a8/libcxx/include/__variant/monostate.h#L32

https://github.com/llvm/llvm-project/pull/195549