[Bug c++/49205] [C++0x] Default constructor with pack expansion parameter not detected

[schaub.johannes at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49205

--- Comment #7 from Johannes Schaub  
2011-06-20 15:56:42 UTC ---
(In reply to comment #6)
> (In reply to comment #1)
> > While this behavior is erroneous, consensus at clang was that WG21 made an
> > oversight in allowing this. Template constructors are banned from being 
> > copy or
> > move constructors, and historically this prohibition was not necessary for
> > default constructors since there was no special handling of them except when
> > implicit.
> 
> I disagree with this.  As Johannes points out, it is possible to have a
> template default constructor in C++03, so changing this would be a significant
> change.  We should just treat the variadic template as a default constructor.

To be fair to Sean, I should note that my example relied on a C++0x feature. If
we remove the template default argument:

template A(T = 0); 

This constructor cannot really be called "with arguments" anymore (there's no
deduction from default arguments), which is the condition under which a
constructor becomes a default constructor.

[Bug c++/49205] [C++0x] Default constructor with pack expansion parameter not detected

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49205

Jason Merrill  changed:

   What|Removed |Added

 Status|ASSIGNED|RESOLVED
 Resolution||FIXED
   Target Milestone|--- |4.7.0

--- Comment #6 from Jason Merrill  2011-06-20 
14:43:23 UTC ---
(In reply to comment #1)
> While this behavior is erroneous, consensus at clang was that WG21 made an
> oversight in allowing this. Template constructors are banned from being copy 
> or
> move constructors, and historically this prohibition was not necessary for
> default constructors since there was no special handling of them except when
> implicit.

I disagree with this.  As Johannes points out, it is possible to have a
template default constructor in C++03, so changing this would be a significant
change.  We should just treat the variadic template as a default constructor.

[Bug c++/49205] [C++0x] Default constructor with pack expansion parameter not detected

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49205

--- Comment #5 from Jason Merrill  2011-06-20 
14:40:15 UTC ---
Author: jason
Date: Mon Jun 20 14:40:10 2011
New Revision: 175214

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=175214
Log:
PR c++/49205
* call.c (sufficient_parms_p): Allow parameter packs too.

Added:
trunk/gcc/testsuite/g++.dg/cpp0x/variadic-default.C
Modified:
trunk/gcc/cp/ChangeLog
trunk/gcc/cp/call.c
trunk/gcc/testsuite/ChangeLog

[Bug c++/49205] [C++0x] Default constructor with pack expansion parameter not detected

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49205

Jason Merrill  changed:

   What|Removed |Added

 Status|UNCONFIRMED |ASSIGNED
   Last reconfirmed||2011.06.20 01:53:05
 CC||jason at gcc dot gnu.org
 AssignedTo|unassigned at gcc dot   |jason at gcc dot gnu.org
   |gnu.org |
 Ever Confirmed|0   |1

[Bug c++/49205] [C++0x] Default constructor with pack expansion parameter not detected

[schaub.johannes at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49205

--- Comment #4 from Johannes Schaub  
2011-05-28 02:06:07 UTC ---
(In reply to comment #3)
> I would expect that the initialization text would be amended appropriately. I
> think that we should go for consistency and say non-templates only, the same 
> as
> for copy and move constructors.

Ah I see. That seems to make sense.

[Bug c++/49205] [C++0x] Default constructor with pack expansion parameter not detected

[scshunt at csclub dot uwaterloo.ca]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49205

--- Comment #3 from Sean Hunt  2011-05-28 
01:55:07 UTC ---
I would expect that the initialization text would be amended appropriately. I
think that we should go for consistency and say non-templates only, the same as
for copy and move constructors.

[Bug c++/49205] [C++0x] Default constructor with pack expansion parameter not detected

[schaub.johannes at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49205

--- Comment #2 from Johannes Schaub  
2011-05-28 01:45:29 UTC ---
(In reply to comment #1)
> While this behavior is erroneous, consensus at clang was that WG21 made an
> oversight in allowing this. Template constructors are banned from being copy 
> or
> move constructors, and historically this prohibition was not necessary for
> default constructors since there was no special handling of them except when
> implicit.

That rationale makes sense. I wonder about the implications for value
initialization though. If that constructor is not a default constructor, then
"A();" appears to be ill-formed, because of the saying in 8.5p7 that we shall
call "the default constructor". 

Also, how should the rules be drawn? Is any template not a default constructor?
Then what about the following?

  template A(T = 0); 

GCC appears to deem it a default constructor. Is the following rule acceptable?

- A default constructor is a constructor with zero parameters or that only has
parameters with default arguments and with an optional trailing ellipsis
("A(int, ...)").