What should alignof do with incomplete types?

[Martin v. Loewis]

> In any case, before preparing a patch, I'd like to know whether the
> current behavior is considered to be a problem.

I haven't seen this bordercase myself, so I guess it is not a problem
for me, either way. However, producing diagnostics appears as the
better solution. I don't care then whether it is an error or a
warning; if it is a warning, the outcome of __alignof__ should be
documented for this case.

Regards,
Martin