[Bug c++/19618] Does warn if bit-fields exceed the size of bool types

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #5 from joseph at codesourcery dot com  2009-02-04 16:30 -------
Subject: Re:  Does warn if bit-fields exceed the size of bool
 types

On Wed, 4 Feb 2009, tydeman at tybor dot com wrote:

> /*
>  * _Bool bit-fields: C99: 6.7.2.1, paragraph 3 along with Defect Report 335.
>  * _Bool bit-fields of size up to CHAR_BIT must be supported.
>  */

I don't know why you think C99 is relevant to a C++ bug report.  As I have 
explained at length before and as we discussed at length at the London 
WG14 meeting, the "width" of a type is the number of value and sign bits, 
so post-TC2 a _Bool:CHAR_BIT bit-field is valid only if the implementation 
defines _Bool to have CHAR_BIT value bits.  GCC defines it to have one 
value bit with the other bits being padding bits and undefined behavior if 
you access a _Bool representation with any of the padding bits having a 
nonzero value (such representations being trap representations).  Thus the 
width of _Bool is 1 with GCC and the diagnostics are required.

As you know, TC2 changed "the number of bits in an object of the type that 
is specified if the colon and expression are omitted" to "the width of an 
object of the type that would be specified were the colon and expression 
omitted". in 6.7.2.1 paragraph 3.

I have no idea what is required by C++ and whether GCC implements the C++ 
requirements correctly or not.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19618