Hi,

>             (long)(&(_prop)->type -&((_struct *)0)->_field)) \

> This seems to be free of gcc extensions and gives more or less sane errors like:
>
> foo.c:26: error: invalid operands to binary - (have ‘long long int *’ and ‘int *’)

The "better error message" bit works.  Nice idea.  Incremental patch 
attached.

cheers,
   Gerd