[Bug c++/16376] New: Bit-field promotions

["jsm28 at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

[conv.prom] paragraph 3 says that bit-fields are converted to int by the
integral promotions if int can represent all the values of the bit-field.

The following testcase was discussed on the WG14 reflector in January
as illustrating a difference between C compilers (where an unsigned:3
bit-field has a special 3-bit type, so is promoted to int) and C++ compilers
(where an unsigned:3 bit-field has type unsigned, per [class.bit]), but
[conv.prom] wasn't mentioned in that discussion and as far as I can tell
it means that C++ compilers too should promote the unsigned:3 bit-field to
int.  If not, what's wrong in my reading of [conv.prom]?

#include <stdio.h>

int main(void){
  struct bits {
    unsigned int ui3 : 3;
  } bits;
  int i = -1;   /* is a very large positive number as unsigned */

  bits.ui3 = 1u;
  if( bits.ui3 < i ){
    (void)printf("ui3 treated as unsigned, i promoted to unsigned\n");
  }else{
    (void)printf("ui3 promoted to int (value preserving)\n");
  }
  return 0;
}

My reading of [conv.prom] is that bits.ui3 should have been promoted to int,
though the statistics from the WG14 reflector were that 29 C++ compilers
converted i to unsigned and only 1 promoted bits.ui3 to int.  There are
still differences between C and C++ for bit-fields of types wider than int.

-- 
           Summary: Bit-field promotions
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jsm28 at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16376