This is something that's come up a number of times: a new POD field is added, but not all the constructors initialize it.

A simple heuristic which should hopefully work is:
If a class is a non-POD, for each constructor, emit a warning for each POD field which does not have an initializer (either in-class or in the initializer list).

I asked about putting a warning like this in clang, and support was lukewarm: in particular, there was concern that this warning would force pointless initialization of large arrays. For now, maybe we can prototype it as a plugin warning.