C++ PATCH: typedefs breaking conversions

[Martin von Loewis]

> Sorry, I meant illegal as in `invokes undefined behaviour'. I
> believe we should emit a warning whenever we can detect undefined
> behaviour that we've not otherwise defined as an extension (1.3.12
> allows us to do this). Andrew's code looked like an application of
> the offsetof macro. That has to work for pod struct, but when
> applied to non-pod, it invokes undefined behaviour (18.1/5).

If you do so, I'd highly appreciate if you would arrange for
predicates that reflect the standard terminology:
- aggregate_p
- pod_struct_p
- pod_union_p
- pod_class_p

Some of that is there, but the description and implementation doesn't
quite match the language in the standard.

Regards,
Martin