c++ - Why do I need to #include <typeinfo> when using the typeid operator?

Question

The typeid represents a C++ RTTI operator being also a C++ keyword. It returns a std::type_info object that holds (dynamic) type specific information.

From what I understood from various sources, one MUST include <typeinfo> when using typeid, otherwise the program is ill-formed. In fact, my gcc5.2 compiler doesn't even compile the program if I don't include the before-mentioned header. I don't understand why is a header inclusion mandated for the usage of a C++ keyword. I understand mandating a header for whenever we use some object declared/defined in that header, but typeid is not of a class type. So what is the reason behind this enforcement of including the header <typeinfo>?

Answer 1

The next paragraph:

The typeid expression is lvalue expression which refers to an object with static storage duration, of the polymorphic type const std::type_info or of some type derived from it.

Because it is an lvalue expression, which uses reference initialization to declare an initializer of std::type_info. <typeinfo> contains the definition for that object.