Add missing assignment operator from 'nullopt_t'

Author: jiixyj

include/beman/optional26/optional.hpp

@@ -299,6 +299,8 @@ class optional {
         requires(!std::is_trivially_destructible_v<T>);
 
     // \ref{optional.assign}, assignment
+    constexpr optional& operator=(nullopt_t) noexcept;
+
     constexpr optional& operator=(const optional& rhs)
         requires std::is_copy_constructible_v<T> && std::is_copy_assignable_v<T> &&
                  (!std::is_trivially_copy_assignable_v<T>);
@@ -514,6 +516,12 @@ inline constexpr optional<T>::~optional()
 
 // 22.5.3.4 Assignment[optional.assign]
 
+template <class T>
+inline constexpr optional<T>& optional<T>::operator=(nullopt_t) noexcept {
+    reset();
+    return *this;
+}
+
 template <class T>
 inline constexpr optional<T>& optional<T>::operator=(const optional<T>& rhs)
     requires std::is_copy_constructible_v<T> && std::is_copy_assignable_v<T> &&

src/beman/optional26/tests/optional.t.cpp

@@ -868,3 +868,23 @@ TEST(OptionalTest, HashTest) {
         EXPECT_EQ(h1, h2);
     }
 }
+
+TEST(OptionalTest, CanHoldValueOfImmovableType) {
+    struct immovable {
+        explicit immovable()                   = default;
+        immovable(const immovable&)            = delete;
+        immovable& operator=(const immovable&) = delete;
+    };
+
+    beman::optional26::optional<immovable> o1(beman::optional26::in_place);
+    EXPECT_TRUE(o1);
+
+    // ...and can reset it with `nullopt`.
+    static_assert(noexcept(o1 = beman::optional26::nullopt));
+    o1 = beman::optional26::nullopt;
+    EXPECT_FALSE(o1);
+
+    // Also, can construct with `nullopt`.
+    beman::optional26::optional<immovable> o2 = beman::optional26::nullopt;
+    EXPECT_FALSE(o2);
+}