[Bug c++/22596] New: Impossible to explicitly instantiate particular overloaded function

[Bug c++/22596] New: Impossible to explicitly instantiate particular overloaded function

[phenning at lanl dot gov]

Compiling the following code with "g++ -c -fno-implict-templates" always generates code for the 
second definition of foo().  It appears that g++ is automatically deducing the template parameters from 
the function parameters, even though the template parameters are explicitly given in the explict 
instantiation.   This problem prevents explicitly instantiating the function

std::_Destroy<int*, std::allocator<int>>(int *, int *, std::allocator<int>);

from bits/stl_construct.h in the 4.x Common Library implementation.

This behavior has been seen in: 
i686-pc-linux-gnu gcc-3.4.2
powerpc-apple-darwin8 gcc-4.0.0 (20041026 Apple Computer build 4061) 
i686-pc-linux-gnu gcc-4.1.0 20050629 (experimental)

// -------------------------------

template<class T> struct A_class
{
  int do_it(int i) { return i+1; }
};

template<class A> int foo(A a)
{
  return a.do_it(1);
}

template<class A, class T> int foo(A_class<T> a)
{
  return a.do_it(2);
}

template int foo< A_class<int> >(A_class<int> a);

// -------------------------------

-- 
           Summary: Impossible to explicitly instantiate particular
                    overloaded function
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: phenning at lanl dot gov
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: i686-pc-linux-gnu, powerpc-apple-darwin8


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[phenning at lanl dot gov]

------- Additional Comments From phenning at lanl dot gov  2005-07-21 18:48 -------
Created an attachment (id=9324)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=9324&action=view)
sample code illustrating problem


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-07-21 18:57 -------
Note both Comeau and ICC have this same behavior.

Even the following code calls the second declaration too:
template<class T> struct A_class
{
  int do_it(int i){return T::f(i);}
};

template<class A> int foo(A a);

template<class A, class T> int foo(A_class<T> a)
{
  return a.do_it(2);
}

void g(void)
{
  A_class<int> a;
  foo<A_class<int> >(a);
}

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-07-21 18:58 -------
I don't think this is a bug in the front-end.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[phenning at lanl dot gov]

------- Additional Comments From phenning at lanl dot gov  2005-07-21 19:16 -------
(In reply to comment #2)

> void g(void)
> {
>   A_class<int> a;
>   foo<A_class<int> >(a);
> }

Right, and I think that it should in that case.  It seems that g++ attempts to call the first definition 
when there is usage like:

foo(something_that_returns_Aclass_int());

So, when I try to compile code that uses std::vector<int>, the compiler generates calls to both 

std::_Destroy<int*, std::allocator<int> >(int *, int*, std::allocator<int>)

and

std::_Destroy<int*, std::allocator<int>, int>(int *, int*, std::allocator<int>)

Since I can't explicitly instantiate the first form, I can't compile std::vector code with -fno-implicit-
templates.  

This could be changed by how _Destroy is implemented, but it seems strange that I can't explicitly 
instantiate a class, given that I'm providing the template parameter signature explicitly.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2005-07-21 23:17 -------
Your explicit instantiation 
  template int foo< A_class<int> >(A_class<int> a); 
obviously matches both the declarations of foo. I'm unsure which 
one the compiler should choose, but if you want to instantiate the 
second one, why don't you write 
  template int foo< A_class<int>,int >(A_class<int> a); 
i.e. explicitly state the second template argument as well? 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[phenning at lanl dot gov]

------- Additional Comments From phenning at lanl dot gov  2005-07-21 23:26 -------
(In reply to comment #5)
> Your explicit instantiation 
>   template int foo< A_class<int> >(A_class<int> a); 
> obviously matches both the declarations of foo. I'm unsure which 
> one the compiler should choose, but if you want to instantiate the 
> second one, why don't you write 
>   template int foo< A_class<int>,int >(A_class<int> a); 
> i.e. explicitly state the second template argument as well? 

The compiler is more than happy to instantiate the second one... the problem is that it is converting my 
one template parameter explicit specialization into the two template parameter form, presumably 
because the function signature matches.  I would expect that if I explicitly state the one parameter 
form, the compiler should instantiate the one parameter form.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2005-07-22 00:18 -------
Oh, I apparently didn't read closely enough. I see the problem now. 
 
For others -- take this: 
--------------------- 
template <typename T> void f(T) {} 
template <typename U, typename T> void f(T) {} 
 
template void f<int> (int); 
--------------------- 
We want the first function to be instantiated, but both functions match 
the signature and gcc (as well as icc) instantiate the second: 
 
g/x> c++ -c x.cc 
g/x> nm -C x.o 
00000000 W void f<int, int>(int) 
 
I remember that there was a PR on this somewhere in bugzilla, and possibly 
a DR on this. I don't remember the conclusion, though... 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
 GCC target triplet|i686-pc-linux-gnu, powerpc- |
                   |apple-darwin8               |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

Jason Merrill <jason at gcc dot gnu.org> changed:

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

--- Comment #8 from Jason Merrill <jason at gcc dot gnu.org> 2011-06-22 16:07:24 UTC ---
The example at the bottom of DR 214 seems related, but not quite the same:

  http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#214

[Bug c++/22596] Impossible to explicitly instantiate particular overloaded function

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22596

Jason Merrill <jason at gcc dot gnu.org> changed:

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

--- Comment #9 from Jason Merrill <jason at gcc dot gnu.org> 2011-06-22 17:35:09 UTC ---
This is not a bug; under the partial ordering rules, the second foo is more
specialized than the first, so if both are possible matches the second will be
chosen.

If you want the explicit instantiation to match the same function that's called
with no explicit template arguments, then don't provide explicit template
arguments in the explicit instantiation.  If you just write

template int foo(A_class<int> a);

or

template int foo<>(A_class<int> a);

it will do what you want.