[As discussed in CWG on 2018-06-07] Consider:

union U { int n; char c; };
U u = {.c = 123};

It is unclear in the current wording that the value representation of u includes all the bits that are part of the n member, not just the bits that are part of the c member, however that is the intent (and the upcoming compare_exchange_* wording depends on that). We should clarify that the set of bits in the value representation depends only on the type, and not on which value of that type we are considering.