Re: typeof and bitfields

[Mark Mitchell <mark at codesourcery dot com>]

Gabriel Dos Reis wrote:
> Mark Mitchell <mark@codesourcery.com> writes:
>
> | Gabriel Dos Reis wrote:
> | > Alexandre Oliva <aoliva@redhat.com> writes:
> | > | On Jan 14, 2005, Gabriel Dos Reis <gdr@integrable-solutions.net>
> | > wrote:
> | > | | > That is an argument for not returning an int.  It is not an
> | > argument
> | > | > for issueing error.  Why not return int_with_2bits?
> | > | | Let's see...
> | 
> | I'm supportive of Joseph's patch.
> | 
> | The submitter in PR10333 clearly thought that you should get an
> | int_with_2bits type.  Matt suggested that you should just get
> | "int". Ian suggested "char".  I see good arguments for all of the
> | choices.  So, there are no obvious semantics.  Why define an extension
> | that the average user has only a 1/3 chance of understanding?
>
> If you take that observation seriously, then you should remove nearly
> all extensions plus at least half of standard language semantics.

That's a reduction-to-absurdity argument.  The standard language 
semantics are not up for debate; some people like them, some people 
don't, but they are what they are.  Some of our extensions are clearly 
documented, and, as such, can be put into a similar category.

This extension was not clearly documented.  A user with a reasonable 
interpretation filed a bug report saying that the compiler was 
misbehaving, when it chose one of the above alternatives.

> | There's only one good reason, and Matt has already given it: backwards
> | compatibility.  Fortunately, that compatibility is only with a GNU
> | extension used in a pretty obscure way, and there is an easy
> | workaround (don't use typeof; use the type of the bitfield instead)
> | that will work in most cases.
>
> The advice "don't use typeof" does not make much sense.  Indeed,
> typeof is mostly used precisely when the type of the operand it not
> known, e.g. in macros. 

I understand the utility of typeof, but lots of people get by without 
using typeof at all.  I'm not going to believe that removing the ability 
to apply typeof to a bitfield is going to cause very many problems for 
very many people.  How many bug reports have we gotten so far?

--
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
(916) 791-8304