[Bug c++/54101] New: Using std::declval for types without a default constructor and with a deleted copy constructor errors.

[supercilious.dude at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54101

             Bug #: 54101
           Summary: Using std::declval for types without a default
                    constructor and with a deleted copy constructor
                    errors.
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: supercilious.d...@gmail.com


GCC rejects code which using std::declval on some types. From what I can
understand from the C++11 standard, such uses of declval are legal inside
decltype.

Clang accepts this without error. GCC 4.7.1 (and the current 4.8 from the
master branch of the git repository today)

Example:

#include <utility>

class NonCopyable
{
  public:
    // Not default constructable.
    NonCopyable ( int )
    {
    }

    NonCopyable ( NonCopyable const& ) = delete;
};


int main ( void )
{
  std::common_type<NonCopyable, NonCopyable>::type t(0);
}

fails with:

In file included from
/home/user/install/VA-HEPHAESTUS/include/c++/4.8.0/bits/move.h:57:0,
                 from
/home/user/install/VA-HEPHAESTUS/include/c++/4.8.0/bits/stl_pair.h:61,
                 from
/home/user/install/VA-HEPHAESTUS/include/c++/4.8.0/utility:72,
                 from gccbug.cxx:1:
/home/user/install/VA-HEPHAESTUS/include/c++/4.8.0/type_traits: In
instantiation of 'struct std::common_type<NonCopyable, NonCopyable>':
gccbug.cxx:16:45:   required from here
/home/user/install/VA-HEPHAESTUS/include/c++/4.8.0/type_traits:1786:29: error:
use of deleted function 'NonCopyable::NonCopyable(const NonCopyable&)'
     { typedef decltype(true ? declval<_Tp>() : declval<_Up>()) type; };
                             ^
gccbug.cxx:10:5: error: declared here
     NonCopyable ( NonCopyable const& ) = delete;
     ^
In file included from
/home/user/install/VA-HEPHAESTUS/include/c++/4.8.0/bits/move.h:57:0,
                 from
/home/user/install/VA-HEPHAESTUS/include/c++/4.8.0/bits/stl_pair.h:61,
                 from
/home/user/install/VA-HEPHAESTUS/include/c++/4.8.0/utility:72,
                 from gccbug.cxx:1:
/home/user/install/VA-HEPHAESTUS/include/c++/4.8.0/type_traits:1786:29: error:
use of deleted function 'NonCopyable::NonCopyable(const NonCopyable&)'
     { typedef decltype(true ? declval<_Tp>() : declval<_Up>()) type; };
                             ^
gccbug.cxx:10:5: error: declared here
     NonCopyable ( NonCopyable const& ) = delete;
     ^