[Bug c++/43663] Can't take a const-ref to a bit field

[Bug c++/43663] Can't take a const-ref to a bit field

[james.dennett at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43663

James Dennett <james.dennett at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |james.dennett at gmail dot
                   |                            |com

--- Comment #6 from James Dennett <james.dennett at gmail dot com> 2011-05-15 11:52:01 UTC ---
Here's a quick hack that causes a temporary to be generated when binding a
bit-field to a reference-to-const.

$ svn diff
Index: call.c
===================================================================
--- call.c    (revision 173769)
+++ call.c    (working copy)
@@ -8594,7 +8594,7 @@
     expr = error_mark_node;
       else
     {
-      if (!lvalue_or_rvalue_with_address_p (expr))
+      if (is_bitfield_expr_with_lowered_type (expr) ||
!lvalue_or_rvalue_with_address_p (expr))
         {
           tree init;
           var = set_up_extended_ref_temp (decl, expr, cleanup, &init);

I'll try to make time to clean that up and add regression tests before running
it by someone with stronger gcc-fu.

[Bug c++/43663] Can't take a const-ref to a bit field

[james.dennett at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43663

--- Comment #7 from James Dennett <james.dennett at gmail dot com> 2011-05-15 11:55:47 UTC ---
Unsurprisingly the quick hack isn't really good enough -- it'll happily bind a
non-const reference to a temporary initialized from a bitfield.  (...and I
guess that's why we have tests, and code reviews.)

[Bug c++/43663] Can't take a const-ref to a bit field

[james.dennett at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43663

--- Comment #8 from James Dennett <james.dennett at gmail dot com> 2011-05-15 12:34:51 UTC ---
Interestingly this works with Apple's g++ 4.2.1, specifically
i686-apple-darwin10-g++-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5666) (dot 3), but
not with their 4.0.1 release.

Tested with:

int main() {
  struct S {
    S(): i(0) {}
    int i : 3;
  };
  S s;
  printf("&s: %p\n", (void*)&s);
  int const& cr(s.i);  // should compile, binding to a temporary
  printf("&cr: %p\n", (void*)&cr);
}

A non-const reference still correctly gives an error:
st.cc:12: error: invalid initialization of reference of type ‘int&’ from
expression of type ‘signed char:3’

[Bug c++/43663] Can't take a const-ref to a bit field

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43663

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|gcc-bugs at gcc dot gnu.org |
   Target Milestone|---                         |4.7.0
      Known to fail|                            |

--- Comment #9 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-11 01:42:19 UTC ---
This is fixed in 4.7.0. I'm adding the testcase and closing the PR.

[Bug c++/43663] Can't take a const-ref to a bit field

[paolo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43663

--- Comment #10 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> 2012-10-11 02:34:39 UTC ---
Author: paolo
Date: Thu Oct 11 02:34:32 2012
New Revision: 192347

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=192347
Log:
2012-10-10  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/43663
    * g++.dg/init/bitfield3.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/init/bitfield3.C
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug c++/43663] Can't take a const-ref to a bit field

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43663

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
      Known to work|                            |4.7.0, 4.8.0
         Resolution|                            |FIXED

--- Comment #11 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-11 02:37:29 UTC ---
Done.

[Bug c++/43663] Can't take a const-ref to a bit field

[jason at gcc dot gnu dot org]

------- Comment #5 from jason at gcc dot gnu dot org  2010-04-07 21:19 -------
Long-standing bug.


-- 

jason at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|4.5.0                       |2.95.4 4.5.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43663

[Bug c++/43663] Can't take a const-ref to a bit field

[redi at gcc dot gnu dot org]

------- Comment #4 from redi at gcc dot gnu dot org  2010-04-06 11:38 -------
Noone's taking the address of a bitfield, the stnadard specifically says "The
address-of operator & shall not be applied to a bit-field, so there are no
pointers to bit-fields."

What happens is the same as:

double d = 0;
const int& i = d;

a temporary is created and the reference bound to it.

And this definitely isn't new in C++0x! The reference for C++03 should be 9.6p3
rather than p2


-- 

redi at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[C++0x] Can't take a const- |Can't take a const-ref to a
                   |ref to a bit field          |bit field


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43663

[Bug c++/43663] Can't take a const-ref to a bit field

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2010-04-06 10:57 -------
Confirmed.  Note that the frontend would need to make sure to unfold the
reference for the middle-end - the middle-end can't bear taking the address
of a bitfield.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |rejects-valid
      Known to fail|                            |4.5.0
   Last reconfirmed|0000-00-00 00:00:00         |2010-04-06 10:57:58
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43663