[Bug c++/41134] New: [4.5 regression] Variable flagges unused if only used in template function

[Bug c++/41134] New: [4.5 regression] Variable flagges unused if only used in template function

[bangerth at gmail dot com]

Here's yet another case of a variable that is flagged as unused when it is
actually referenced. I agree that this is a corner case: the variable 
really *is* unused, the template function is not instantiated after all.
That said, this is not an uncommon idiom, the code is extracted from the
Threading Building Blocks library.

-------------------------------
static int i;
template <typename> int f() { return i; }
----------------------------------

deal.II> c++ -W -Wunused -c x.cc
x.cc:1:12: warning: 'i' defined but not used


W.


-- 
           Summary: [4.5 regression] Variable flagges unused if only used in
                    template function
           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=41134

[Bug c++/41134] [4.5 regression] Variable flagges unused if only used in template function

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2009-08-20 20:44 -------
Well this is invalid code that is accepted by GCC anyways ....


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41134

[Bug c++/41134] [4.5 regression] Variable flagged unused if only used in template function

[bangerth at gmail dot com]

------- Comment #2 from bangerth at gmail dot com  2009-08-20 20:47 -------
(In reply to comment #1)
> Well this is invalid code that is accepted by GCC anyways ....

How so??


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41134

[Bug c++/41134] [4.5 regression] Variable flagged unused if only used in template function

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2009-08-20 20:49 -------
You cannot use a static variable in a template :).
Note the following is valid though:
namespace { int i; }
template <typename> int f() { return i; }

And still invokes the warning:
t4.cc:1:17: warning: '<unnamed>::i' defined but not used


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41134

[Bug c++/41134] [4.5 regression] Variable flagged unused if only used in template function

[bangerth at gmail dot com]

------- Comment #4 from bangerth at gmail dot com  2009-08-20 20:54 -------
(In reply to comment #3)
> You cannot use a static variable in a template :).

I would be unaware of that restriction. It's true that you can't use objects
with internal linkage (such as static variables) as *template arguments* but
I would be surprised if you couldn't use them in the body of a template
function. That said, this point is tangential to what I wanted to say with
this PR, and your little programs makes my point as eloquently as my original
one ;-)

W.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41134

[Bug c++/41134] [4.5 regression] Variable flagged unused if only used in template function

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2009-08-20 21:11 -------
Why is this a regression?  Which compiler version didn't warn here?

The question is really whether we want unused variables just in the case
where we can remove it and without further changes the program is still
valid or if we want to detect more elaborate cases where the programmer
can remove a variable with some extra work.


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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=41134

[Bug c++/41134] [4.5 regression] Variable flagged unused if only used in template function

[bangerth at gmail dot com]

------- Comment #6 from bangerth at gmail dot com  2009-08-20 21:20 -------
(In reply to comment #5)
> Why is this a regression?  Which compiler version didn't warn here?

4.3.2. Current mainline warns.


> The question is really whether we want unused variables just in the case
> where we can remove it and without further changes the program is still
> valid or if we want to detect more elaborate cases where the programmer
> can remove a variable with some extra work.

Right, and I don't have the answer. All I'm saying is: this is, in a nutshell,
one of the header files of the threading building blocks. I used to be able
to compile my program with that file included with -W -Wunused and not get
a warning, but now I do get a warning if I happen to not use the template
function that is defined in the header file.

W.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41134

[Bug c++/41134] [4.5 regression] Variable flagged unused if only used in template function

[jason at gcc dot gnu dot org]

------- Comment #7 from jason at gcc dot gnu dot org  2009-08-21 21:47 -------


*** This bug has been marked as a duplicate of 41109 ***


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jason at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41134

[Bug c++/41134] [4.5 regression] Variable flagged unused if only used in template function

[jason at gcc dot gnu dot org]

------- Comment #8 from jason at gcc dot gnu dot org  2009-08-24 21:32 -------
Subject: Bug 41134

Author: jason
Date: Mon Aug 24 21:31:54 2009
New Revision: 151061

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=151061
Log:
        PR c++/41109
        PR c++/41110
        PR c++/41134
        * cp-tree.h (DECL_ODR_USED): New macro.
        (struct lang_decl_base): Add odr_used flag.
        * decl.c (duplicate_decls): Propagate it.  Use it for error.
        * pt.c (register_specialization): Use it for error.
        * decl2.c (mark_used): Use it as gating flag rather than TREE_USED.
        (cp_write_global_declarations): Use it for error.
        (tree_used_ok): Remove.
        * cp-tree.h: Remove tree_used_ok.
        * call.c (build_call_a): Don't call it.
        * init.c (build_offset_ref): Likewise.

Added:
    trunk/gcc/testsuite/g++.dg/warn/Wunused-17.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/call.c
    trunk/gcc/cp/cp-tree.h
    trunk/gcc/cp/decl.c
    trunk/gcc/cp/decl2.c
    trunk/gcc/cp/init.c
    trunk/gcc/cp/pt.c
    trunk/gcc/testsuite/ChangeLog


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41134