Implement std::hash for beman::optional

A std::hash implementation for optional, the hash of the underlying value, or 0
for disengaged.

Author: steve-downey

include/Beman/Optional26/optional.hpp

@@ -1147,4 +1147,21 @@ class optional<T&> {
 
 } // namespace beman::optional26
 
+namespace std {
+template <typename T>
+    requires requires(T a) {
+        { std::hash<remove_const_t<T>>{}(a) } -> std::convertible_to<std::size_t>;
+    }
+struct hash<beman::optional26::optional<T>> {
+    static_assert(!is_reference_v<T>, "hash is not enabled for reference types");
+    size_t operator()(const beman::optional26::optional<T>& o) const
+        noexcept(noexcept(hash<remove_const_t<T>>{}(*o))) {
+        if (o) {
+            return std::hash<std::remove_const_t<T>>{}(*o);
+        } else {
+            return 0;
+        }
+    }
+};
+} // namespace std
 #endif // BEMAN_OPTIONAL26_OPTIONAL_HPP

src/Beman/Optional26/tests/optional.t.cpp

@@ -596,6 +596,21 @@ TEST(OptionalTest, RangeTest) {
     }
 }
 
+TEST(OptionalTest, HashTest) {
+    beman::optional26::optional<int> o1 = beman::optional26::nullopt;
+    beman::optional26::optional<int> o2 = beman::optional26::nullopt;
+    beman::optional26::optional<int> o3 = 42;
+    beman::optional26::optional<int> o4 = 42;
+
+    auto h1 = std::hash<beman::optional26::optional<int>>{}(o1);
+    auto h2 = std::hash<beman::optional26::optional<int>>{}(o2);
+    auto h3 = std::hash<beman::optional26::optional<int>>{}(o3);
+    auto h4 = std::hash<beman::optional26::optional<int>>{}(o4);
+
+    EXPECT_EQ(h1, h2);
+    EXPECT_EQ(h3, h4);
+}
+
 TEST(ViewMaybeTest, Constructors) {
     std::ranges::single_view<std::optional<int>> s;
     std::ranges::single_view<std::optional<int>> s2{s};