[libcxx-commits] [libcxx] Make std::nullopt_t comparable (PR #195549)

[A. Jiang via libcxx-commits]

================
@@ -362,6 +362,15 @@ struct nullopt_t {
     explicit __secret_tag() = default;
   };
   _LIBCPP_HIDE_FROM_ABI constexpr explicit nullopt_t(__secret_tag, __secret_tag) noexcept {}
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator==(const nullopt_t&, const nullopt_t) { return true; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator!=(const nullopt_t&, const nullopt_t) { return false; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator>(const nullopt_t&, const nullopt_t) { return false; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator<(const nullopt_t&, const nullopt_t) { return false; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator>=(const nullopt_t&, const nullopt_t) { return true; }
+  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator<=(const nullopt_t&, const nullopt_t) { return true; }
+#    if _LIBCPP_STD_VER >= 20
+  _LIBCPP_HIDE_FROM_ABI constexpr friend strong_ordering operator<=>(const nullopt_t&, const nullopt_t&) = default;
+#    endif
----------------
frederick-vs-ja wrote:

We can have less operator functions in C++20 and later. Note that `operator==` can be implicitly declared and defined if a defaulted `operator<=>` is declared.
```suggestion
#    if _LIBCPP_STD_VER >= 20
  _LIBCPP_HIDE_FROM_ABI constexpr friend strong_ordering operator<=>(const nullopt_t&, const nullopt_t&) = default;
#    else
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator==(const nullopt_t&, const nullopt_t) { return true; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator!=(const nullopt_t&, const nullopt_t) { return false; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator>(const nullopt_t&, const nullopt_t) { return false; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator<(const nullopt_t&, const nullopt_t) { return false; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator>=(const nullopt_t&, const nullopt_t) { return true; }
  _LIBCPP_HIDE_FROM_ABI constexpr friend bool operator<=(const nullopt_t&, const nullopt_t) { return true; }
#    endif
```

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