Re: [boost] clang failing core/detail_iterator_test?

Subject: Re: [boost] clang failing core/detail_iterator_test?
From: Peter Dimov (lists_at_[hidden])
Date: 2014-06-06 17:53:14

Andrey Semashev wrote:

> I can reproduce it locally with clang 3.4. Shows only 
> with -std=c++11 -stdlib=libc++. Trying to dig further.

Ah. Sneaky hobbits. Conforming extension my posterior. :-)

(Actually, it _is_ a conforming extension, by a strict reading of the 
standard. Still, I'd have preferred a little less cleverness.)

template <class _Tp>
struct __has_iterator_category
{
private:
    struct __two {char __lx; char __lxx;};
    template <class _Up> static __two __test(...);
    template <class _Up> static char __test(typename _Up::iterator_category* 
= 0);
public:
    static const bool value = sizeof(__test<_Tp>(0)) == 1;
};

template <class _Iter, bool> struct __iterator_traits_impl {};

template <class _Iter>
struct __iterator_traits_impl<_Iter, true>
{
    typedef typename _Iter::difference_type   difference_type;
    typedef typename _Iter::value_type        value_type;
    typedef typename _Iter::pointer           pointer;
    typedef typename _Iter::reference         reference;
    typedef typename _Iter::iterator_category iterator_category;
};

template <class _Iter, bool> struct __iterator_traits {};

template <class _Iter>
struct __iterator_traits<_Iter, true>
    :  __iterator_traits_impl
      <
        _Iter,
        is_convertible<typename _Iter::iterator_category, 
input_iterator_tag>::value ||
        is_convertible<typename _Iter::iterator_category, 
output_iterator_tag>::value
      >
{};

// iterator_traits<Iterator> will only have the nested types if 
Iterator::iterator_category
//    exists.  Else iterator_traits<Iterator> will be an empty class.  This 
is a
//    conforming extension which allows some programs to compile and behave 
as
//    the client expects instead of failing at compile time.

template <class _Iter>
struct _LIBCPP_TYPE_VIS_ONLY iterator_traits
    : __iterator_traits<_Iter, __has_iterator_category<_Iter>::value> {};