[Bug c++/24702] New: Koenig found functoid ref, but "cannot be used as a function"

[Bug c++/24702] New: Koenig found functoid ref, but "cannot be used as a function"

[pierhyth at gmail dot com]

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The problem relates to functoid objects ("function like" objects, ie
ones with an operator() defined) and references to them within a 
namespace.  When Koenig lookup is used to find such a 
reference-to-functoid, g++ seems to find it okay, but then states 
"cannot be used as a function".  If I use an actual functoid object 
instead of a reference to one, it works fine.

Here is an example:

#include <iostream>

namespace dummyx {

  struct Dummy {
    Dummy() {}
  };

  struct DummyFunct {
    int operator()(Dummy d) const {return 5;}
    static DummyFunct& full() {static DummyFunct f; return f;}
  };
  static DummyFunct& dummyFunct=DummyFunct::full();

  DummyFunct myDummyFunct;

  DummyFunct& mymyDummyFunct=myDummyFunct;

}

int main() {

  ::dummyx::Dummy const d;

  std::cout<<"dummyx::dummyFunct(d) = "<<dummyx::dummyFunct(d)<<std::endl;
  std::cout<<"dummyFunct(d) = "<<dummyFunct(d)<<std::endl; // fails with 3.4
and 4.0
  std::cout<<"myDummyFunct(d) = "<<myDummyFunct(d)<<std::endl;
  std::cout<<"mymyDummyFunct(d) = "<<mymyDummyFunct(d)<<std::endl; // fails
with 3.4 and 4.0

}

When I try to compile this code using g++ version 4.0.2 I get the
following errors:

testit.cc: In function ‘int main()’:
testit.cc:26: error: ‘dummyx::dummyFunct’ cannot be used as a function
testit.cc:28: error: ‘dummyx::mymyDummyFunct’ cannot be used as a function

I see no reason why such references should not be able to be used
as a function.  As you see, if it is explicitly namespace-qualified
it can be.  And according to Koenig rules, the unqualified name should
indeed be found and, as operator() is applicable for the found reference,
it should be usable as a function.  Indeed, this is what version 3.2.3
of g++ does.

The above test code compiles fine with g++ version 3.2.3.  The errors
seem only to occur in versions 3.4 and later (certainly they occur for 
versions 3.4.5 and 4.0.2.)

Additional version information:

  g++ (GCC) 3.2.3 20030502 (Red Hat Linux 3.2.3-20)

  g++-3.4 (GCC) 3.4.5 20050809 (prerelease) (Ubuntu 3.4.4-6ubuntu8)

  g++-4.0 (GCC) 4.0.2 20050808 (prerelease) (Ubuntu 4.0.1-4ubuntu9)


-- 
           Summary: Koenig found functoid ref, but "cannot be used as a
                    function"
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pierhyth at gmail dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2005-11-07 13:36 -------
Actually IIRC Koenig lookup only finds functions and not variables.  If that is
the case we have two issues here.  One is we accept invalid and another is that
error message is wrong.

Note ICC rejects this:
t.cc(26): error: identifier "dummyFunct" is undefined
    std::cout<<"dummyFunct(d) = "<<dummyFunct(d)<<std::endl; // fails with 3.4
                                   ^

t.cc(28): error: identifier "myDummyFunct" is undefined
    std::cout<<"myDummyFunct(d) = "<<myDummyFunct(d)<<std::endl;
                                     ^

t.cc(29): error: identifier "mymyDummyFunct" is undefined
    std::cout<<"mymyDummyFunct(d) = "<<mymyDummyFunct(d)<<std::endl; // fails
                                       ^


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[pierhyth at gmail dot com]

------- Comment #2 from pierhyth at gmail dot com  2005-11-09 01:33 -------
The current C++ standard says that Koenig applies "when an unqualified 
name is used as the postfix-expression in a function call" which from
what I've read, must include the case of functoids/functors and references
to functoids/functors.

It is true that some people seem to be arguing that the standard be changed
to restrict Koenig to only functions, a view which seems to me rather
unfortunate.  However the fact remains that the current standard allows
functors and the g++ compiler complies with this by allowing functors.
The issue is that references to functors appear to be broken in the most
recent versions of g++.

I have written much code that relies on g++ finding references to functors
through Koenig lookup.  This code compiled fine with version 3.2.  The
problem only arose when I tried to update to the 4.0 compiler.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[gdr at integrable-solutions dot net]

------- Comment #3 from gdr at integrable-solutions dot net  2005-11-16 19:28 -------
Subject: Re:  Koenig found functoid ref, but "cannot be used as a function"

"pinskia at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| Actually IIRC Koenig lookup only finds functions and not variables. 

That is incorrect.  There is an active core language issue on
"argument dependent name lookup specification."

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[gdr at gcc dot gnu dot org]

------- Comment #4 from gdr at gcc dot gnu dot org  2005-12-04 05:09 -------
this issue should be resolved one way of the other based on the
core issue about argument dependent lookup specification, when a
non-function is found.  The "obvious" solution would be to do overload
resolution based on the signatures (whether actual function, constructor, 
function objects, or references to those).  However, all that needs
tracking the open core issue.


-- 

gdr at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gdr at gcc dot gnu dot org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-12-04 05:09:46
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|gcc-bugs at gcc dot gnu.org        |

--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Gaby, can you help me with this old issue? I can't find any DR still open in
this area, and we behave exactly like current clang & icc, that is we reject
the last three lines (about unqualified dummyFunct, myDummyFunct, and
mymyDummyFunct). Can we resolve this bug?

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The standard says:

When considering an associated namespace, the lookup is the same as the lookup
performed when the associated namespace is used as a qualifier (3.4.3.2) except
that:
- [...]
- [...]
- All names except those of (possibly overloaded) functions and function
templates are ignored.

So non-functions are ignored.

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#218 for the
rationale.

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

--- Comment #8 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Thanks Jon!

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[pierhyth at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

--- Comment #9 from Mark Phillips <pierhyth at gmail dot com> ---
Thanks Jonathan for the information on the updated standard and the rationale
behind restricting Koenig to actual functions and function templates.

In one way it is a bit of a pity - it does make functors second-class citizens
compared to functions - but the linked document explains some additional
considerations, namely the risk of over-visibility of names.  Perhaps it is a
reasonable choice for now.  Hopefully a better solution will be introduced down
the track (maybe extending it to functors, while at the same time
narrowing/allowing-control-over namespaces to be looked up).

[Bug c++/24702] Koenig found functoid ref, but "cannot be used as a function"

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24702

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I wouldn't hold your breath unfortunately. There are at least two opposing
views on how to "fix" ADL within the standard committee and no consensus on
what (if anything) should be done.