为什么clang认为gcc的子范围不满足gcc的__ranges_begin函数概念要求？

Question

clang 失败的代码（而 gcc 似乎没问题）

    int arr[] { 111, 222, 333};
    ranges::subrange(
        ranges::begin(arr),ranges::end(arr) );

看起来 clang 声称 gcc 的子范围没有 begin 函数？！

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/bits/iterator_concepts.h:953:35: note: and 'std::ranges::subrange<int *, int *, std::ranges::subrange_kind::sized> &' does not satisfy '__member_begin'

      requires is_array_v<_Tp> || __member_begin<_Tp&> || __adl_begin<_Tp&>

                                  ^

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/bits/iterator_concepts.h:937:33: note: because '__detail::__decay_copy(__t.begin())' would be invalid: no member named 'begin' in 'std::ranges::subrange<int *, int *, std::ranges::subrange_kind::sized>'

          { __detail::__decay_copy(__t.begin()) } -> input_or_output_iterator;

                                       ^

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/bits/iterator_concepts.h:953:59: note: and 'std::ranges::subrange<int *, int *, std::ranges::subrange_kind::sized> &' does not satisfy '__adl_begin'

      requires is_array_v<_Tp> || __member_begin<_Tp&> || __adl_begin<_Tp&>

[直播]

---

gcc 的 iterator_concepts.h 的一部分：

   namespace __detail
   {
     template<typename _Tp>
       constexpr decay_t<_Tp>
       __decay_copy(_Tp&& __t)
       noexcept(is_nothrow_convertible_v<_Tp, decay_t<_Tp>>)
       { return std::forward<_Tp>(__t); }
  
     template<typename _Tp>
       concept __member_begin = requires(_Tp& __t)
         {
           { __detail::__decay_copy(__t.begin()) } -> input_or_output_iterator;
         };
  
     void begin(auto&) = delete;
     void begin(const auto&) = delete;
  
     template<typename _Tp>
       concept __adl_begin = __class_or_enum<remove_reference_t<_Tp>>
         && requires(_Tp& __t)
         {
           { __detail::__decay_copy(begin(__t)) } -> input_or_output_iterator;
         };
  
     // Simplified version of std::ranges::begin that only supports lvalues,
     // for use by __range_iter_t below.
     template<typename _Tp>
       requires is_array_v<_Tp> || __member_begin<_Tp&> || __adl_begin<_Tp&>
       auto
       __ranges_begin(_Tp& __t)
       {
         if constexpr (is_array_v<_Tp>)
           {
             static_assert(sizeof(remove_all_extents_t<_Tp>) != 0,
                           "not array of incomplete type");
             return __t + 0;
           }
         else if constexpr (__member_begin<_Tp&>)
           return __t.begin();
         else
           return begin(__t);
       }
  
     // Implementation of std::ranges::iterator_t, without using ranges::begin.
     template<typename _Tp>
       using __range_iter_t
         = decltype(__detail::__ranges_begin(std::declval<_Tp&>()));
  
   } // namespace __detail

Answer 1

正如问题评论中提到的，现在这个问题已经由

Clang 16

和

libc++

和

stdlibc++

解决了

[直播]