<html>
    <head>
      <base href="http://llvm.org/bugs/" />
    </head>
    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW --- - Error in std::pair default constructor when elements are not default-constructible"
   href="http://llvm.org/bugs/show_bug.cgi?id=21917">21917</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Error in std::pair default constructor when elements are not default-constructible
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>libc++
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>3.4
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>PC
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Linux
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>normal
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>All Bugs
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>eniebler@boost.org
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvmbugs@cs.uiuc.edu, mclow.lists@gmail.com
          </td>
        </tr>

        <tr>
          <th>Classification</th>
          <td>Unclassified
          </td>
        </tr></table>
      <p>
        <div>
        <pre>The following trivial code causes a hard error:

-------------------------------------
#include <utility>
#include <tuple>

int main()
{
    int i=0,j=0;
    std::make_tuple(std::make_pair(std::ref(i),std::ref(j)));
}
-------------------------------------

The error is:

-------------------------------------
/usr/include/c++/v1/utility:259:58: error: reference to type 'int' requires an
initializer
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR pair() : first(), second() {}
                                                         ^
/usr/include/c++/v1/type_traits:2342:38: note: in instantiation of member
function
      'std::__1::pair<int &, int &>::pair' requested here
    : public integral_constant<bool, __is_constructible(_Tp, _Args...)>
                                     ^
/usr/include/c++/v1/type_traits:2646:14: note: in instantiation of template
class
      'std::__1::is_constructible<std::__1::pair<int &, int &>>' requested here
    : public is_constructible<_Tp>
             ^
/usr/include/c++/v1/tuple:515:26: note: in instantiation of template class
      'std::__1::is_default_constructible<std::__1::pair<int &, int &> >'
requested here
        __all<(_Dummy && is_default_constructible<_Tp>::value)...>::value
                         ^
/home/eric/libcxx-test/test.cpp:7:5: note: in instantiation of template class
      'std::__1::tuple<std::__1::pair<int &, int &> >' requested here
    std::make_tuple(std::make_pair(std::ref(i),std::ref(j)));
    ^
In file included from /home/eric/libcxx-test/test.cpp:1:
/usr/include/c++/v1/utility:259:67: error: reference to type 'int' requires an
initializer
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR pair() : first(), second() {}
                                                                  ^
2 errors generated.
-------------------------------------


Tuple is asking pair if it is default-constructible (I'm unsure why), and just
asking is triggering the error because pair is *not* default-constructible. But
the default constructor exists anyway.

This is most certainly a std defect, but IMO libc++ should be proactive and
SFINAE's pair's default constructor if _T1 and _T2 are not
default-constructible. (Defining it =default would be a behavior change because
it wouldn't value-initialize it's members when the pair is
default-initialized.)

Here is a simple struct that shows how the pair default constructor could be
defined:

-------------------------------------------
template<typename T1, typename T2>
struct my_pair
{
    T1 first;
    T2 second;
    template<
    bool B = true,
    typename = typename std::enable_if<
        std::is_default_constructible<T1>::value &&
        std::is_default_constructible<T2>::value &&
        B
    >::type
    >
    my_pair() :first(), second()
    {}
    my_pair(const T1& f, const T2& s)
    :first(f), second(s)
    {}
};
-------------------------------------------</pre>
        </div>
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