diff --git a/source/mijin/container/boxed_object.hpp b/source/mijin/container/boxed_object.hpp
index 2adc357..c131991 100644
--- a/source/mijin/container/boxed_object.hpp
+++ b/source/mijin/container/boxed_object.hpp
@@ -32,13 +32,14 @@ class BoxedObject
 {
 private:
     union {
+        std::byte placeholder_;
         T object_;
     };
 #if MIJIN_BOXED_OBJECT_DEBUG
     bool constructed = false;
 #endif
 public:
-    BoxedObject() = default;
+    BoxedObject() noexcept : placeholder_() {};
     explicit BoxedObject(T object)
 #if MIJIN_BOXED_OBJECT_DEBUG
     : constructed(true)
@@ -59,6 +60,11 @@ public:
     BoxedObject& operator=(const BoxedObject&) = delete;
     BoxedObject& operator=(BoxedObject&&) = delete;
 
+    T& operator*() noexcept { return get(); }
+    const T& operator*() const noexcept { return get(); }
+    T* operator->() noexcept { return &get(); }
+    const T* operator->() const noexcept { return &get(); }
+
     template<typename... TArgs>
     void construct(TArgs&&... args)
     {
diff --git a/source/mijin/container/optional.hpp b/source/mijin/container/optional.hpp
index 46304c3..54ae8ed 100644
--- a/source/mijin/container/optional.hpp
+++ b/source/mijin/container/optional.hpp
@@ -138,6 +138,10 @@ public:
     constexpr explicit operator bool() const noexcept { return !empty(); }
     [[nodiscard]]
     constexpr bool operator !() const noexcept { return empty(); }
+    [[nodiscard]]
+    constexpr std::remove_reference_t<TValue>& operator*() noexcept { return get(); }
+    [[nodiscard]]
+    constexpr const std::remove_reference_t<TValue>& operator*() const noexcept { return get(); }
 public:
     template<typename... Types>
     void emplace(Types&&... params) noexcept;
diff --git a/source/mijin/debug/assert.hpp b/source/mijin/debug/assert.hpp
index 44bea03..12ff2db 100644
--- a/source/mijin/debug/assert.hpp
+++ b/source/mijin/debug/assert.hpp
@@ -28,7 +28,7 @@ namespace mijin
 
 #if MIJIN_DEBUG
 
-#define MIJIN_DO_RAISE_ERROR(msg, source_loc) \
+#define MIJIN_RAISE_ERROR(msg, source_loc) \
 switch (mijin::handleError(msg, source_loc))  \
 {                                             \
     case mijin::ErrorHandling::CONTINUE:      \
@@ -40,10 +40,8 @@ switch (mijin::handleError(msg, source_loc))  \
         std::abort();                         \
 }
 
-#define MIJIN_RAISE_ERROR(msg, func, file, line) MIJIN_DO_RAISE_ERROR(msg, func, file, line)
-
 #define MIJIN_ERROR(msg) \
-MIJIN_DO_RAISE_ERROR(msg, std::source_location::current())
+MIJIN_RAISE_ERROR(msg, std::source_location::current())
 
 #define MIJIN_FATAL(msg) \
 MIJIN_ERROR(msg)         \
@@ -57,7 +55,7 @@ if (!static_cast<bool>(condition))
     if (true) /*!ignoreAll */                                                      \
     {                                                                              \
         const mijin::AssertionResult assertion_result__ = mijin::handleAssert(     \
-                #condition, msg, MIJIN_FUNC(), __FILE__, __LINE__);                \
+                #condition, msg, std::source_location::current());                 \
         switch (assertion_result__)                                                \
         {                                                                          \
         case mijin::AssertionResult::ABORT:                                        \
@@ -116,20 +114,17 @@ enum class ErrorHandling
 
 #ifdef MIJIN_USE_CUSTOM_ASSERTION_HANDLER
 AssertionResult handleAssert(const char* condition,
-        const char* message, const char* function,
-        const char* file, int line) noexcept;
+        const char* message, const std::source_location& location);
 #else
 constexpr AssertionResult handleAssert(const char* /* condition */,
-        const char* /* message */, const char* /* function */,
-        const char* /* file */, int /* line */)
+        const char* /* message */, const std::source_location& /* location */)
 {
     return AssertionResult::ERROR;
 }
 #endif // MIJIN_USE_CUSTOM_ASSERTION_HANDLER
 
 #ifdef MIJIN_USE_CUSTOM_ERROR_HANDLER
-ErrorHandling handleError(const char* message, const char* function,
-        const char* file, int line) noexcept;
+ErrorHandling handleError(const char* message, const std::source_location& location);
 #else
 inline ErrorHandling handleError(const char* message, const std::source_location& location) noexcept
 {
diff --git a/source/mijin/util/iterators.hpp b/source/mijin/util/iterators.hpp
index e0be6cd..2aff82a 100644
--- a/source/mijin/util/iterators.hpp
+++ b/source/mijin/util/iterators.hpp
@@ -10,6 +10,7 @@
 #include <string_view>
 #include <tuple>
 #include <variant>
+#include "../container/optional.hpp"
 
 namespace mijin
 {
@@ -277,7 +278,7 @@ struct MappingIterator
 {
     using orig_value_type = typename std::iterator_traits<TIterator>::value_type;
     using difference_type = std::ptrdiff_t;
-    using value_type = std::invoke_result_t<TFunctor, orig_value_type>;
+    using value_type = std::invoke_result_t<TFunctor, orig_value_type&>;
     using reference = value_type&;
     using iterator_category = std::bidirectional_iterator_tag; // TODO?
 
@@ -285,7 +286,7 @@ struct MappingIterator
 
     TIterator base;
     TFunctor functor;
-    mutable std::optional<value_type> result;
+    mutable Optional<value_type> result;
 
     MappingIterator(TIterator base_, TFunctor functor_) noexcept : base(base_), functor(std::move(functor_)) {}
     MappingIterator(const MappingIterator&) noexcept = default;
@@ -296,8 +297,8 @@ struct MappingIterator
 
     reference operator*() const noexcept
     {
-        if (!result.has_value()) {
-            result = functor(*base);
+        if (result.empty()) {
+            result = std::invoke(functor, *base);
         }
         return *result;
     }
@@ -305,7 +306,7 @@ struct MappingIterator
     MappingIterator& operator++() noexcept
     {
         ++base;
-        result = std::nullopt;
+        result.reset();
         return *this;
     }
 
@@ -319,7 +320,7 @@ struct MappingIterator
     MappingIterator& operator--() noexcept
     {
         --base;
-        result = std::nullopt;
+        result.reset();
         return *this;
     }