[Bug c++/43393] integral promotion of long bit-fields broken in g++ 4.4.0

["vsoni at tilera dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #3 from vsoni at tilera dot com  2010-03-17 19:33 -------
(In reply to comment #2)
> 
> I read that t.f promotes to int.  And that is exactly what the C++ frontend
> does:

That's plausible, but the standard, especially it's intent, is unclear I think.

I see three plausible interpretations:

A) int can represent all values of a long:16 bit-field, AND a long:16 bit-field
is not larger than int (if "larger" is interpreted with respect to the number
of bits in the bit-field representation, i.e. 16 bits).  So it is converted to
int.

B) int can represent all values of a long:16 bit-field, BUT a long:16 bit-field
is larger than int and unsigned int (if "larger" is interpreted with respect to
the size of the declared type of the bit-field, i.e. long).  So two clauses of
[3] apply, and the question is order of precedence:

  B1) The clause concerning int-representable values takes precedence, so it is
converted to int.

  B2) The clause concerning the size of the bit-field takes precedence, so no
integer promotions apply, and it remains long.

I agree that [A] seems to be the "simplest" interpretation in some sense, and
perhaps it's the correct one, but I can't reconcile this with the following: 
It seems to me the programmer's intent when declaring a bit-field of type
long:N, is to treat the value as type long, independent of N.  It also seems to
me that's the intent of the standard when it states that the number of bits in
a bit-field is not part of its type; perhaps the intent is something else?

FWIW, I searched but did not find a discussion of rationale, or a conclusive
example, in the supplementary notes of the C++ Standards Committee.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43393