[Bug libstdc++/113522] New: std::swap cannot be called with explicit template argument std::array<int,3>

[Bug libstdc++/113522] New: std::swap cannot be called with explicit template argument std::array<int,3>

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113522

            Bug ID: 113522
           Summary: std::swap cannot be called with explicit template
                    argument std::array<int,3>
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This program

#include <array>
#include <utility>

int main() {
    std::array<int,3> a, b;
    std::swap< std::array<int,3> >( a, b ); 
}

is accepted by MSVC and Clang with libc++, but GCC with libstdc++ complains:

error: no matching function for call to 'swap<std::array<int, 3>
>(std::array<int, 3>&, std::array<int, 3>&)'

Online demo: https://godbolt.org/z/8xnd5PsfE

Related discussion: https://stackoverflow.com/q/77844837/7325599

[Bug libstdc++/113522] std::swap cannot be called with explicit template argument std::array<int,3>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113522

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is silly code and there is no reason to support it.

[Bug libstdc++/113522] std::swap cannot be called with explicit template argument std::array<int,3>

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113522

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It was added in r7-5068-ga2863bde755d39 for LWG 2766 .

[Bug libstdc++/113522] std::swap cannot be called with explicit template argument std::array<int,3>

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113522

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://timsong-cpp.github.io/lwg-issues/2766

The LWG issue is still open ...

[Bug libstdc++/113522] std::swap cannot be called with explicit template argument std::array<int,3>

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113522

Jiang An <de34 at live dot cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |de34 at live dot cn

--- Comment #4 from Jiang An <de34 at live dot cn> ---
std::swap is not supposed to be called with explicitly specified template
arguments.

If the template arguments are not explicitly specified for std::swap, the
correct overloads will be selected if you include currect headers, even when
not all standard headers that declare std::swap overloads are included (the
shape of overload set is unspecified in such cases).

However, if you explicitly specify template arguments, there may be unintended
(likely incorrect) overloads whose template arguments are no longer
underdeduced to be selected. And then undesired implicit conversion can be
trigged during overload resolution, which can make your program ill-formed.

(In reply to Jonathan Wakely from comment #1)
> This is silly code and there is no reason to support it.

Hmm... currently it's specified in [algorithms.requirements] that
> The well-formedness and behavior of a call to an algorithm with an explicitly-specified template argument list is unspecified, except where explicitly stated otherwise.

(Added by P0896R4 and amended by LWG3419.)

Should open an LWG issue to specify similar things for std::swap?

[Bug libstdc++/113522] std::swap cannot be called with explicit template argument std::array<int,3>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113522

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jiang An from comment #4)
> Hmm... currently it's specified in [algorithms.requirements] that
> > The well-formedness and behavior of a call to an algorithm with an explicitly-specified template argument list is unspecified, except where explicitly stated otherwise.
> 
> (Added by P0896R4 and amended by LWG3419.)
> 
> Should open an LWG issue to specify similar things for std::swap?

Yes please. I thought we already had it, but when I went looking I only found
that text in [algorithm.requirements].

[Bug libstdc++/113522] std::swap cannot be called with explicit template argument std::array<int,3>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113522

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'd also like to ban it for std::make_pair, but that would break loads of very
silly code that does std::make_pair<T,U>(a,b) (c.f. Bug 92300 comment 3).

[Bug libstdc++/113522] std::swap cannot be called with explicit template argument std::array<int,3>

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113522

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #6)
> I'd also like to ban it for std::make_pair, but that would break loads of
> very silly code that does std::make_pair<T,U>(a,b) (c.f. Bug 92300 comment
> 3).

Note GCC/go has that code too:

go/gofrontend/expressions.cc:    return std::make_pair<Call_expression*,
Temporary_statement*>(NULL, NULL);
go/gofrontend/expressions.cc:    return std::make_pair<Call_expression*,
Temporary_statement*>(NULL, NULL);
go/gofrontend/expressions.cc:  return std::make_pair<Call_expression*,
Temporary_statement*>(NULL, NULL);
go/gofrontend/import.cc:  this->type_offsets_.resize(maxp1,
std::make_pair<size_t, size_t>(0, 0));

:)

[Bug libstdc++/113522] std::swap cannot be called with explicit template argument std::array<int,3>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113522

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-01-30
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |SUSPENDED

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
https://cplusplus.github.io/LWG/issue4047