[Bug c++/39875] New: [4.5 regression] Wrong "value computed is not used" warning

[Bug c++/39875] New: [4.5 regression] Wrong "value computed is not used" warning

[bangerth at gmail dot com]

I think this is from after 4.4 branched:
----------------------
template <typename TT> struct InputIterator
{
    InputIterator ()
      {
        TT i;
        (void)*i;           // require dereference operator
      }
};

InputIterator<int *> i;
---------------------

> c++ -c deal.II/source/dofs/dof_renumbering.cc  -W -Wall
deal.II/source/dofs/dof_renumbering.cc: In constructor
'InputIterator<TT>::InputIterator() [with TT = int*]':
deal.II/source/dofs/dof_renumbering.cc:10:   instantiated from here
deal.II/source/dofs/dof_renumbering.cc:6: warning: value computed is not used

That's obviously bogus, the compiler warns about a statement that is
explicitly cast to 'void' so there is no computed value any more.

The code is culled from boost's concepts checks, so this is going to hit
quite a number of people.

W.


-- 
           Summary: [4.5 regression] Wrong "value computed is not used"
                    warning
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bangerth at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39875

[Bug c++/39875] [4.5 regression] Wrong "value computed is not used" warning

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.5.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39875

[Bug c++/39875] [4.5 regression] Wrong "value computed is not used" warning

[bangerth at gmail dot com]

------- Comment #1 from bangerth at gmail dot com  2009-04-26 17:10 -------
May be related to PR 39889.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39875

[Bug c++/39875] [4.5 regression] Wrong "value computed is not used" warning

[jakub at gcc dot gnu dot org]

------- Comment #2 from jakub at gcc dot gnu dot org  2009-04-26 19:23 -------
No need for templates:
void foo ()
{
  int *i;
  (void) *i; // { dg-bogus "value computed is not used" }
}
This was caused by
http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=146132
(PR39551).  convert_to_void doesn't know if the conversion to void was caused
by explicit cast (in this testcase, no warning should be emitted) or implicit
(when it should be emitted).


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lcwu at gcc dot gnu dot org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2009-04-26 19:23:28
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39875

[Bug c++/39875] [4.5 regression] Wrong "value computed is not used" warning

[jakub at gcc dot gnu dot org]

------- Comment #3 from jakub at gcc dot gnu dot org  2009-04-26 19:36 -------
Created an attachment (id=17732)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17732&action=view)
gcc45-pr39875.patch

Actually, convert_to_void knows that, see the implicit argument.


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |jakub at gcc dot gnu dot org
                   |dot org                     |
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39875

[Bug c++/39875] [4.5 regression] Wrong "value computed is not used" warning

[jakub at gcc dot gnu dot org]

------- Comment #4 from jakub at gcc dot gnu dot org  2009-04-27 05:54 -------
Subject: Bug 39875

Author: jakub
Date: Mon Apr 27 05:54:32 2009
New Revision: 146825

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=146825
Log:
        PR c++/39875
        * cvt.c (convert_to_void) <case INDIRECT_REF>: Only warn about
        -Wunused-value if implicit.

        * g++.dg/warn/Wunused-15.C: New test.

Added:
    trunk/gcc/testsuite/g++.dg/warn/Wunused-15.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/cvt.c
    trunk/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39875

[Bug c++/39875] [4.5 regression] Wrong "value computed is not used" warning

[jakub at gcc dot gnu dot org]

------- Comment #5 from jakub at gcc dot gnu dot org  2009-04-27 05:55 -------
Fixed.


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39875