type_info: a default constructible type isn't a good idea actually

docs/md/core.md

@@ -559,9 +559,8 @@ require to enable RTTI.<br/>
 Therefore, they can sometimes be even more reliable than those obtained
 otherwise.
 
-Its type defines a default constructible opaque class that is also copyable and
-movable.<br/>
-Objects of this class are generally returned by the `type_id` functions:
+Its type defines an opaque class that is also copyable and movable.<br/>
+Objects of this type are generally returned by the `type_id` functions:
 
 ```cpp
 // by type
@@ -571,11 +570,16 @@ auto info = entt::type_id<a_type>();
 auto other = entt::type_id(42);
 ```
 
-The objects thus received are nothing more than const references to instances of
-`type_info` with static storage duration.<br/>
+All elements thus received are nothing more than const references to instances
+of `type_info` with static storage duration.<br/>
 This is convenient for saving the entire object aside for the cost of a pointer.
-However, nothing prevents from constructing `type_info` objects directly.<br/>
-These are the information made available by this kind of objects:
+However, nothing prevents from constructing `type_info` objects directly:
+
+```cpp
+entt::type_info info{std::in_place_type<int>};
+```
+
+These are the information made available by `type_info`:
 
 * The index associated with a given type:
 

src/entt/core/type_info.hpp

@@ -21,8 +21,7 @@ namespace internal {
 struct ENTT_API type_index final {
     [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
         static ENTT_MAYBE_ATOMIC(id_type) value{};
-        // 0u is reserved for empty type_info objects
-        return ++value;
+        return value++;
     }
 };
 
@@ -140,12 +139,6 @@ struct type_name final {
 
 /*! @brief Implementation specific information about a type. */
 struct type_info final {
-    /*! @brief Default constructor. */
-    constexpr type_info() ENTT_NOEXCEPT
-        : seq{},
-          identifier{},
-          alias{} {}
-
     /**
      * @brief Constructs a type info object for a given type.
      * @tparam Type Type for which to construct a type info object.
@@ -154,9 +147,7 @@ struct type_info final {
     constexpr type_info(std::in_place_type_t<Type>) ENTT_NOEXCEPT
         : seq{type_index<std::remove_cv_t<std::remove_reference_t<Type>>>::value()},
           identifier{type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value()},
-          alias{type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()} {
-        ENTT_ASSERT(seq != 0u, "Invalid type index");
-    }
+          alias{type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()} {}
 
     /**
      * @brief Type index.
@@ -182,14 +173,6 @@ struct type_info final {
         return alias;
     }
 
-    /**
-     * @brief Returns true if properly initialized, false otherwise.
-     * @return True if properly initialized, false otherwise.
-     */
-    [[nodiscard]] explicit constexpr operator bool() const ENTT_NOEXCEPT {
-        return (seq != 0u);
-    }
-
 private:
     id_type seq;
     id_type identifier;

test/entt/core/type_info.cpp

@@ -84,16 +84,6 @@ TEST(TypeInfo, Functionalities) {
     ASSERT_EQ(other.name(), info.name());
 }
 
-TEST(TypeInfo, Validity) {
-    entt::type_info info{};
-    entt::type_info other = entt::type_id<int>();
-
-    ASSERT_FALSE(info);
-    ASSERT_TRUE(other);
-    ASSERT_EQ(info.index(), 0u);
-    ASSERT_NE(other.index(), 0u);
-}
-
 TEST(TypeInfo, Order) {
     entt::type_info rhs = entt::type_id<int>();
     entt::type_info lhs = entt::type_id<char>();