template <class Derived>
class Base1
{
public:
  Base()
  {
     static_cast<Derived*>(this)->type_tag = 1;
  }
};

class Derived: public Base<Derived>
{
public:
  int type_tag;
};

_W>template <class Derived>
_W>class Base1
_W>{
_W>public:
_W>  Base()
_W>  {
_W>     static_cast<Derived*>(this)->type_tag = 1;
_W>  }
_W>};

_W>class Derived: public Base<Derived>
_W>{
_W>public:
_W>  int type_tag;
_W>};
_W>

// Грязный хак:

// код для Base

class Derived: public Base<Derived>
{
public:
  Derived() {} // отсутсвует инициализация type_tag;

  int type_tag;
};

_W>Но интересно, не запрещает ли стандарт делать static_cast к наследнику, если он еще не сконструирован. Чисто академический интерес

5.2.9/8
An rvalue of type “pointer to cv1 B”, where B is a class type, can be converted to an rvalue of type “pointer
to cv2 D”, where D is a class derived (clause 10) from B.... If the rvalue of type “pointer to cv1 B” points to a B that is actually a sub-object of an object of type D, the resulting pointer points to the enclosing object of type D. Otherwise, the result of the
cast is undefined.

3.8/1
The lifetime of an object is a runtime property of the object. The lifetime of an object of type T begins
when:
— storage with the proper alignment and size for type T is obtained, and
— if T is a class type with a non-trivial constructor (12.1), the constructor call has completed.

3.8/5
Before the lifetime of an object has started but after the storage which the object will occupy has been allocated ... If the object will be or was of a non-POD class type, the program has
undefined behavior if:
— the pointer is used to access a non-static data member or call a non-static member function of the object,