Are there cases where a typedef is absolutely necessary?

Question

Consider the following excerpt from the safe bool idiom:

typedef void (Testable::*bool_type)() const;
operator bool_type() const;

Is it possibl

Answer 1

Answering the "Are there cases where a typedef is absolutely necessary?" from the question title, here is one example of where a typedef is needed:

f(unsigned char());   // compiler error!
typedef unsigned char Byte;
f(Byte());            // fine!

See the results here: http://ideone.com/JPUra

Answer 2

Ah, I just remembered the identity meta-function. It is possible to write

operator typename identity<void (Testable::*)() const>::type() const;

with the following definition of identity:

template <typename T>
struct identity
{
    typedef T type;
};

You could argue that identity still uses a typedef, but this solution is "good" enough for me.