Proposed Resolution (September, 2008): Change 11.4 [class.mem] paragraph 18 as follows:
... Two standard-layout structs share a common initial sequence
if corresponding members have layout-compatible types
(and,
for bit-fields, the same widths) and either neither member
is a bit-field or both are bit-fields with the same widths
for a sequence of one or more initial members.
The current wording defining a “common initial sequence”
in 11.4 [class.mem] paragraph 17 does not address the case
in which one member is a bit-field and the corresponding member is
not: Presumably the intent was something like, “(and, if one
of the pair is a bit-field, the other is also a bit-field of the
same width).”
Two standard-layout structs share a common initial sequence if
corresponding members have layout-compatible types (and, for
bit-fields, the same widths) for a sequence of one or more initial
members.