[LLVMbugs] [Bug 21917] New: Error in std::pair default constructor when elements are not default-constructible

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21917

            Bug ID: 21917
           Summary: Error in std::pair default constructor when elements
                    are not default-constructible
           Product: libc++
           Version: 3.4
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: eniebler at boost.org
                CC: llvmbugs at cs.uiuc.edu, mclow.lists at gmail.com
    Classification: Unclassified

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)
    {}
};
-------------------------------------------

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