I looked it up for C, and as far as I can tell this has not standardized. (The Defect Report I read, which seems to have the latest opinion, literally calls such things "wobbly values". I'm not joking.) The TL;DR of Defect Report #451 seems to be that this is needs some work from the standard. In the case of an automatic struct or whatever (so you can't pull the bytes out from under it and reuse it) I think the consensus seems to be tending towards Heisenburg-ish values that have values that can change between reads, and certain library functions will be legal to call on them so that they "collapse" the state in the view that you've just created. For your case, I would have the question of whether two objects (e.g. a char array and a struct carved from that array) can share the same memory; in this case I would intuitively tend to agree with you that the padding bits are visible from the longer-lived object, kind of like an ad-hoc union. But I am not a licensed language lawyer, so I am not quite sure on that point.

I appreciate your edits, by the way ;)