Is it possible to deduce whether type is incomplete without compilation failure?

Question

I want to achieve behavior like sizeof(complete_type) will return real sizeof, and sizeof(incomplete_type) - will be just 0

I need this to provide extended run time

Answer 1

Do not try to do that.

It is fundamentally unsound. Templates are parametrized by types, not instantiation point. A class type is not complete or not in itself, it is complete at some point during translation.

A template instantiated on some types must have the exact same semantic in every instantiation.

If not, the behaviour is not defined.