[Bug c++/37256] New: extern template / explicit instantiation broken in 4.4.0-pre

[Bug c++/37256] New: extern template / explicit instantiation broken in 4.4.0-pre

[ciaran dot mccreesh at googlemail dot com]

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The following C++ code compiles (but doesn't link) with g++ 4.3.1:

    template <typename T_>
    struct B
    {
        T_ f();
    };

    extern template class B<int>;

    void f()
    {
        B<int> t;
        t.f();
    }

With 'gcc () 4.4.0-pre9999 built 20080827 (Gentoo SVN ebuild) rev. 139623', the
following error occurs:

$ g++ -O1 a.cc 
a.cc: In instantiation of ‘T_ B<T_>::f() [with T_ = int]’:
a.cc:12:   instantiated from here
a.cc:12: error: explicit instantiation of ‘T_ B<T_>::f() [with T_ = int]’ but
no definition available

If no -O is specified, the error does not occur. Using -std=c++0x does not
alter the outcome.

I'm looking at n2723 14.7.2 for what the behaviour should be. My understanding
is as follows: By paragraph 7, the explicit instantiation declaration for
B<int> is also an explicit instantiation declaration for int B<int>::f(). Thus,
by paragraph 3, a definition of B<int> must be available when declaring int
B<int>::f(), which it is. But I don't see a requirement for a definition of
B<int>::f().


-- 
           Summary: extern template / explicit instantiation broken in
                    4.4.0-pre
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ciaran dot mccreesh at googlemail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] extern template / explicit instantiation broken in 4.4.0-pre

[paolo dot carlini at oracle dot com]

------- Comment #1 from paolo dot carlini at oracle dot com  2008-08-27 16:33 -------
Just a clarification about the "broken this, broken that" way of putting
things: G++ started providing the extern template syntax *many*, and I mean it,
*many* years ago, well before the standardization in C++0x. AFAICS, this
testcase never linked. Thus, if anything, it's just that now, with C++0x, we
have a divergence between the behavior of extern template as a G++ extension
and its behavior as mandated by n2723. Nothing can be broken.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] extern template / explicit instantiation broken in 4.4.0-pre

[sebor at roguewave dot com]

------- Comment #2 from sebor at roguewave dot com  2008-08-27 16:48 -------
Is this by any chance related to bug 24511?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] extern template / explicit instantiation broken in 4.4.0-pre

[paolo dot carlini at oracle dot com]

------- Comment #3 from paolo dot carlini at oracle dot com  2008-08-27 16:53 -------
Maybe you right Martin, I don't have at hand anything older than 4.0.4 ;)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] extern template / explicit instantiation broken in 4.4.0-pre

[ciaran dot mccreesh at googlemail dot com]

------- Comment #4 from ciaran dot mccreesh at googlemail dot com  2008-08-27 17:07 -------
I don't expect it to link. I do expect it to compile. In real code I would of
course have struct B in a header and template class B<int>; in a different
source file.

So far as I can see this is a change in behaviour in 4.4 that goes against both
the draft standard and how the GCC extension used to work, and which makes
'extern template' effectively useless.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] extern template / explicit instantiation broken in 4.4.0-pre

[paolo dot carlini at oracle dot com]

------- Comment #5 from paolo dot carlini at oracle dot com  2008-08-27 17:17 -------
Ok, *your* issue is about current mainline only, and only -O and above. Let's
CC Jason...


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paolo dot carlini at oracle dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] [4.4 Regression] extern template / explicit instantiation broken in 4.4.0-pre

[paolo dot carlini at oracle dot com]

------- Comment #6 from paolo dot carlini at oracle dot com  2008-08-27 19:36 -------
And to be safe, let's mark it as Regression. Sorry for not having noticed the
issue about -O earlier.


-- 

paolo dot carlini at oracle dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2008-08-27 19:36:05
               date|                            |
            Summary|extern template / explicit  |[4.4 Regression] extern
                   |instantiation broken in     |template / explicit
                   |4.4.0-pre                   |instantiation broken in
                   |                            |4.4.0-pre


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] [4.4 Regression] extern template / explicit instantiation broken in 4.4.0-pre

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2008-08-27 19:38 -------
I want to say this is caused by one of the unit at a time changes ...


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org
           Keywords|                            |rejects-valid
   Target Milestone|---                         |4.4.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] [4.4 Regression] extern template / explicit instantiation broken in 4.4.0-pre

[jsm28 at gcc dot gnu dot org]

-- 

jsm28 at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] [4.4 Regression] extern template / explicit instantiation broken in 4.4.0-pre

[jason at gcc dot gnu dot org]

-- 

jason at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |jason at gcc dot gnu dot org
                   |dot org                     |
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2008-08-27 19:36:05         |2008-11-19 00:10:15
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] [4.4 Regression] extern template / explicit instantiation broken in 4.4.0-pre

[jason at gcc dot gnu dot org]

------- Comment #8 from jason at gcc dot gnu dot org  2008-11-19 19:29 -------
Subject: Bug 37256

Author: jason
Date: Wed Nov 19 19:27:29 2008
New Revision: 142014

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=142014
Log:
        PR c++/37256
        * pt.c (instantiate_decl): Don't require a definition of
        a template that is explicitly instantiated 'extern'.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/extern_template-3.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/pt.c
    trunk/gcc/testsuite/ChangeLog


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256

[Bug c++/37256] [4.4 Regression] extern template / explicit instantiation broken in 4.4.0-pre

[jason at gcc dot gnu dot org]

------- Comment #9 from jason at gcc dot gnu dot org  2008-11-19 19:41 -------
Fixed.


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jason at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37256