The C++ specification generally does not rely on the C core language specification. (The C standard library is incorporated by reference, but the C++ core language specification is standalone.) The purpose of this reference to C is to point out the intentional difference between the two languages. This should be clarified if it's causing confusion.

I think it is a bit confusing -- it reads a bit like "padding bits cannot have traps, see C for rationale". Maybe it could be clarified with:

[ Note: Padding bits have unspecified value, but do not cause traps. See also <ins><xref>Annex C</xfref> and </ins>ISO C 6.2.6.2. — end note ]

and add a compatibility entry to Annex C.1 since this is a difference in behaviors between the two languages?

He did, he said [basic.fundamental]p4.

@AaronBallman, Annex C.1 lists situations where a valid C program may change semantics or be ill-formed when compiled as C++. The situation here is not such a case, since we are changing from trap (undefined behavior) to well-defined behavior.

We currently say

[Note: Padding bits have unspecified value, but do not cause traps. See also ISO C 6.2.6.2. -- end note]

Should we instead say "In contrast, see ISO C 6.2.6.2." ?

@AaronBallman, Annex C.1 lists situations where a valid C program may change semantics or be ill-formed when compiled as C++. The situation here is not such a case, since we are changing from trap (undefined behavior) to well-defined behavior.

Okay, that's a fair point.

Should we instead say "In contrast, see ISO C 6.2.6.2." ?

That would clarify it for me, yes.