[Bug c++/58713] error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’

["daniel.kruegler at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58713

--- Comment #6 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Daniel Krügler from comment #5)
> Thanks for your test, Marc. I will reflect upon the problem in a bit more
> detail

My current guess is that my suggested approach should work, assuming a proper
helper trait template will be used. Here is the model code that I used for
testing:

//---------------------------
#include <type_traits>
#include <utility>

namespace xstd {

template<class CharT = char>
struct OStream 
{
  OStream& operator<<(int);
  OStream& operator<<(short);
  OStream& operator<<(double);
  OStream& operator<<(const void*);
};

struct is_ostreamable_impl 
{
  template<class CharT, class T, class =
    decltype(std::declval<OStream<CharT>&>() << std::declval<T>())>
  static std::true_type test(int);

  template<class, class>
  static std::false_type test(...);
};

template<class CharT, class T>
struct is_ostreamable 
  : decltype(is_ostreamable_impl::test<CharT, T>(0))::type
{  
};

template<class CharT, class T,
  typename std::enable_if<
    is_ostreamable<CharT, const T&>::value, 
    bool
  >::type = false>
  OStream<CharT>& operator<<(OStream<CharT>&& os, const T& t) 
{
  return os << t;
}

OStream<> out;

}

struct A {};

int main() {
  A a;
  xstd::out << 1;
  xstd::out << a;
}
//---------------------------

The diagnostics of my gcc current 4.9.0 is

<quote>
main.cpp||In function 'int main()':|
main.cpp|50|error: no match for 'operator<<' (operand types are
'xstd::OStream<>' and 'A')|
main.cpp|50|note: candidates are:|
main.cpp|9|note: xstd::OStream<CharT>& xstd::OStream<CharT>::operator<<(int)
[with CharT = char]|
main.cpp|9|note:   no known conversion for argument 1 from 'A' to 'int'|
main.cpp|10|note: xstd::OStream<CharT>& xstd::OStream<CharT>::operator<<(short
int) [with CharT = char]|
main.cpp|10|note:   no known conversion for argument 1 from 'A' to 'short int'|
main.cpp|11|note: xstd::OStream<CharT>&
xstd::OStream<CharT>::operator<<(double) [with CharT = char]|
main.cpp|11|note:   no known conversion for argument 1 from 'A' to 'double'|
main.cpp|12|note: xstd::OStream<CharT>& xstd::OStream<CharT>::operator<<(const
void*) [with CharT = char]|
main.cpp|12|note:   no known conversion for argument 1 from 'A' to 'const
void*'|
main.cpp|36|note: template<class CharT, class T, typename
std::enable_if<xstd::is_ostreamable<CharT, const T&>::value, bool>::type
<anonymous> > xstd::OStream<CharT>& xstd::operator<<(xstd::OStream<CharT>&&,
const T&)|
main.cpp|36|note:   template argument deduction/substitution failed:|
main.cpp|35|error: no type named 'type' in 'struct std::enable_if<false,
bool>'|
main.cpp|35|note: invalid template non-type parameter|
</quote>
>From gcc-bugs-return-431655-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Mon Oct 14 13:18:50 2013
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Subject: [Bug libfortran/58015] FAIL: gfortran.dg/round_4.f90: Unsatisfied symbol "nextafterl"
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idX015

Rainer Orth <ro at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|hppa64-hp-hpux11.11         |hppa64-hp-hpux11.11,
                   |                            |*-*-solaris2.9
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-10-14
                 CC|                            |ro at gcc dot gnu.org
               Host|hppa64-hp-hpux11.11         |hppa64-hp-hpux11.11,
                   |                            |*-*-solaris2.9
   Target Milestone|---                         |4.9.0
     Ever confirmed|0                           |1
              Build|hppa64-hp-hpux11.11         |hppa64-hp-hpux11.11,
                   |                            |*-*-solaris2.9

--- Comment #5 from Rainer Orth <ro at gcc dot gnu.org> ---
Also on Solaris 9, both SPARC and x86.

  Rainer