diff --git a/Siv3D/include/Siv3D/Optional.hpp b/Siv3D/include/Siv3D/Optional.hpp
index 5fddeeee9..fc6c0217b 100644
--- a/Siv3D/include/Siv3D/Optional.hpp
+++ b/Siv3D/include/Siv3D/Optional.hpp
@@ -95,16 +95,22 @@ namespace s3d
 		constexpr value_type value_or_eval(Fty&& f)&&;
 
 		template <class Fty, std::enable_if_t<std::is_invocable_v<Fty, value_type&>>* = nullptr>
-		constexpr void then(Fty&& f);
+		constexpr void then(Fty&& f)&;
 
 		template <class Fty, std::enable_if_t<std::is_invocable_v<Fty, value_type>>* = nullptr>
-		constexpr void then(Fty&& f) const;
+		constexpr void then(Fty&& f) const&;
+
+		template <class Fty, std::enable_if_t<std::is_invocable_v<Fty, value_type>>* = nullptr>
+		constexpr void then(Fty&& f)&&;
 
 		template <class Fty, class R = std::decay_t<std::invoke_result_t<Fty, value_type&>>>
-		Optional<R> map(Fty f);
+		Optional<R> map(Fty f)&;
+
+		template <class Fty, class R = std::decay_t<std::invoke_result_t<Fty, value_type>>>
+		Optional<R> map(Fty f) const&;
 
 		template <class Fty, class R = std::decay_t<std::invoke_result_t<Fty, value_type>>>
-		Optional<R> map(Fty f) const;
+		Optional<R> map(Fty f)&&;
 
 		template <class CharType>
 		friend std::basic_ostream<CharType>& operator <<(std::basic_ostream<CharType>& output, const Optional<Type>& value)
diff --git a/Siv3D/include/Siv3D/detail/Optional.ipp b/Siv3D/include/Siv3D/detail/Optional.ipp
index 673e85296..98452f8cb 100644
--- a/Siv3D/include/Siv3D/detail/Optional.ipp
+++ b/Siv3D/include/Siv3D/detail/Optional.ipp
@@ -122,7 +122,14 @@ namespace s3d
 	[[nodiscard]]
 	inline constexpr typename Optional<Type>::value_type Optional<Type>::value_or_construct(Args&&... args) const&
 	{
-		return static_cast<bool>(*this) ? **this : Type(std::forward<Args>(args)...);
+		if (*this)
+		{
+			return **this;
+		}
+		else
+		{
+			return Type(std::forward<Args>(args)...);
+		}
 	}
 
 	template <class Type>
@@ -130,26 +137,47 @@ namespace s3d
 	[[nodiscard]]
 	inline constexpr typename Optional<Type>::value_type Optional<Type>::value_or_construct(Args&&... args)&&
 	{
-		return static_cast<bool>(*this) ? std::move(**this) : Type(std::forward<Args>(args)...);
+		if (*this)
+		{
+			return std::move(**this);
+		}
+		else
+		{
+			return Type(std::forward<Args>(args)...);
+		}
 	}
 
 	template <class Type>
 	template <class Fty, std::enable_if_t<std::conjunction_v<std::is_copy_constructible<Type>, std::is_convertible<std::invoke_result_t<Fty>, Type>>>*>
 	inline constexpr typename Optional<Type>::value_type Optional<Type>::value_or_eval(Fty&& f) const&
 	{
-		return static_cast<bool>(*this) ? **this : std::forward<Fty>(f)();
+		if (*this)
+		{
+			return **this;
+		}
+		else
+		{
+			return std::forward<Fty>(f)();
+		}
 	}
 
 	template <class Type>
 	template <class Fty, std::enable_if_t<std::conjunction_v<std::is_move_constructible<Type>, std::is_convertible<std::invoke_result_t<Fty>, Type>>>*>
 	inline constexpr typename Optional<Type>::value_type Optional<Type>::value_or_eval(Fty&& f)&&
 	{
-		return static_cast<bool>(*this) ? std::move(**this) : std::forward<Fty>(f)();
+		if (*this)
+		{
+			return std::move(**this);
+		}
+		else
+		{
+			return std::forward<Fty>(f)();
+		}
 	}
 
 	template <class Type>
 	template <class Fty, std::enable_if_t<std::is_invocable_v<Fty, Type&>>*>
-	inline constexpr void Optional<Type>::then(Fty&& f)
+	inline constexpr void Optional<Type>::then(Fty&& f)&
 	{
 		if (has_value())
 		{
@@ -159,7 +187,7 @@ namespace s3d
 
 	template <class Type>
 	template <class Fty, std::enable_if_t<std::is_invocable_v<Fty, Type>>*>
-	inline constexpr void Optional<Type>::then(Fty&& f) const
+	inline constexpr void Optional<Type>::then(Fty&& f) const&
 	{
 		if (has_value())
 		{
@@ -167,9 +195,19 @@ namespace s3d
 		}
 	}
 
+	template <class Type>
+	template <class Fty, std::enable_if_t<std::is_invocable_v<Fty, Type>>*>
+	inline constexpr void Optional<Type>::then(Fty&& f)&&
+	{
+		if (has_value())
+		{
+			std::forward<Fty>(f)(std::move(*this).value());
+		}
+	}
+
 	template <class Type>
 	template <class Fty, class R>
-	inline Optional<R> Optional<Type>::map(Fty f)
+	inline Optional<R> Optional<Type>::map(Fty f)&
 	{
 		if (has_value())
 		{
@@ -183,7 +221,7 @@ namespace s3d
 
 	template <class Type>
 	template <class Fty, class R>
-	inline Optional<R> Optional<Type>::map(Fty f) const
+	inline Optional<R> Optional<Type>::map(Fty f) const&
 	{
 		if (has_value())
 		{
@@ -195,6 +233,20 @@ namespace s3d
 		}
 	}
 
+	template <class Type>
+	template <class Fty, class R>
+	inline Optional<R> Optional<Type>::map(Fty f)&&
+	{
+		if (has_value())
+		{
+			return f(std::move(*this).value());
+		}
+		else
+		{
+			return none;
+		}
+	}
+
 	template <class Type1, class Type2>
 	inline constexpr bool operator ==(const Optional<Type1>& lhs, const Optional<Type2>& rhs)
 	{