Add implicit conversions for box from derived to base type

asl/box.hpp

@@ -35,6 +35,13 @@ public:
         , m_alloc{ASL_MOVE(other.m_alloc)}
     {}
 
+    template<is_object U>
+    requires convertible_from<T*, U*>
+    constexpr box(box<U, Allocator>&& other) // NOLINT(*-explicit-conversions)
+        : m_ptr{exchange(other.m_ptr, nullptr)}
+        , m_alloc{ASL_MOVE(other.m_alloc)}
+    {}
+
     constexpr box& operator=(box&& other)
     {
         if (this == &other) { return *this; }
@@ -93,6 +100,9 @@ public:
 
     template<is_object U, allocator A>
     friend constexpr U* leak(box<U, A>&&);
+
+    template<is_object U, allocator A>
+    friend class box;
 };
 
 template<is_object T, allocator Allocator = DefaultAllocator, typename... Args>

asl/tests/box_tests.cpp

@@ -76,3 +76,27 @@ ASL_TEST(niche)
     ASL_TEST_EXPECT(!opt2.has_value());
     ASL_TEST_EXPECT(destroyed);
 }
+
+class Base
+{
+public:
+    virtual ~Base() = default;
+    virtual int number() { return 1; }
+};
+
+class Derived : public Base
+{
+public:
+    int number() override { return 2; }
+};
+
+static_assert(asl::convertible_from<asl::box<Base>, asl::box<Derived>>);
+static_assert(asl::convertible_from<asl::box<Base>, asl::box<Base>>);
+static_assert(!asl::convertible_from<asl::box<Derived>, asl::box<Base>>);
+static_assert(!asl::convertible_from<asl::box<int>, asl::box<float>>);
+
+ASL_TEST(derived)
+{
+    asl::box<Base> obj = asl::make_box<Derived>();
+    ASL_TEST_ASSERT(obj->number() == 2);
+}