build: add EnTT library

include/entt/meta/policy.hpp

@@ -0,0 +1,81 @@
+#ifndef ENTT_META_POLICY_HPP
+#define ENTT_META_POLICY_HPP
+
+#include "../stl/type_traits.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+struct meta_policy {};
+
+} // namespace internal
+/*! @endcond */
+
+/*! @brief Empty class type used to request the _as-is_ policy. */
+struct as_value_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename>
+    static constexpr bool value = true;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as void_ policy. */
+struct as_void_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename>
+    static constexpr bool value = true;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as ref_ policy. */
+struct as_ref_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename Type>
+    static constexpr bool value = stl::is_reference_v<Type> && !stl::is_const_v<stl::remove_reference_t<Type>>;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as cref_ policy. */
+struct as_cref_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename Type>
+    static constexpr bool value = stl::is_reference_v<Type>;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as auto_ policy. */
+struct as_is_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename>
+    static constexpr bool value = true;
+    /*! @endcond */
+};
+
+/**
+ * @brief Provides the member constant `value` equal to true if a type also is a
+ * meta policy, false otherwise.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+struct is_meta_policy
+    : stl::bool_constant<stl::is_base_of_v<internal::meta_policy, Type>> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+inline constexpr bool is_meta_policy_v = is_meta_policy<Type>::value;
+
+/**
+ * @brief Specifies whether a type is a meta policy.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+concept meta_policy = is_meta_policy_v<Type>;
+
+} // namespace entt
+
+#endif