Implement Fix for LWG 4497 as adopted at Croydon

include/beman/optional/optional.hpp

@@ -122,6 +122,13 @@ struct nullopt_t {
      * @details constexpr for nullopt_t to be literal.
      */
     explicit constexpr nullopt_t(Tag) noexcept {}
+
+  private:
+    friend constexpr bool operator==(nullopt_t, nullopt_t) noexcept { return true; }
+
+    friend constexpr std::strong_ordering operator<=>(nullopt_t, nullopt_t) noexcept {
+        return std::strong_ordering::equivalent;
+    }
 };
 
 /// Tag to disengage optional objects.

tests/beman/optional/optional_range_support.test.cpp

@@ -436,3 +436,20 @@ TEST(RangeSupportTest, PythagoreanTriples) {
     EXPECT_TRUE(constify(lambda()));
     EXPECT_TRUE(lambda());
 }
+
+TEST(RangeSupportTest, NullOpt) {
+    beman::optional::nullopt_t t1{beman::optional::nullopt_t::Tag::tag};
+    beman::optional::nullopt_t t2{beman::optional::nullopt_t::Tag::tag};
+    EXPECT_EQ(t1, t2);
+    EXPECT_LE(t1, t2);
+    EXPECT_LE(t2, t1);
+    EXPECT_EQ(t1, beman::optional::nullopt);
+}
+
+TEST(RangeSupportTest, FindNullopt) {
+    std::vector<beman::optional::optional<int>> v;
+
+    auto i = std::ranges::find(v, beman::optional::nullopt);
+
+    EXPECT_EQ(i, std::ranges::end(v));
+}