Structured bindings do not work with `environment::key_value_pair[_view]`

[vovams]

Structured bindings do not work with environment::key_value_pair and environment::key_value_pair_view. For example, the following code does not compile when using GCC 15.2.1 or Clang 21.1.6 on Linux:

#include <iostream>
#include <boost/process/environment.hpp>

int main() {
    for (const auto [name, value] : boost::process::environment::current())
        std::cout << name << '=' << value << '\n';
    return 0;
}

Compiler output

$ g++ bug.cpp -lboost_process
bug.cpp: In function ‘int main()’:
bug.cpp:5:21: error: cannot decompose inaccessible member ‘boost::process::v2::environment::key_value_pair_view::value_’ of ‘const boost::process::v2::environment::key_value_pair_view’
    5 |     for (const auto [name, value] : boost::process::environment::current())
      |                     ^~~~~~~~~~~~~
In file included from /usr/include/boost/process/environment.hpp:1,
                 from bug.cpp:2:
/usr/include/boost/process/v2/environment.hpp:435:20: note: declared private here
  435 |   string_view_type value_;
      |                    ^~~~~~
$ clang bug.cpp -lboost_process
bug.cpp:5:21: error: type 'const key_value_pair_view' decomposes into 1 element, but 2 names were
      provided
    5 |     for (const auto [name, value] : boost::process::environment::current())
      |                     ^
/usr/include/boost/process/v2/environment.hpp:1377:14: note: selected 'begin' function with
      iterator type 'iterator'
 1377 |     iterator begin() const {return iterator(handle_.get());}
      |              ^
1 error generated.

Interestingly, if const auto is replaced with just auto, GCC compiles the code without any warnings, while Clang still does not compile it but produces a different error message, that seems to explain the actual cause:

bug.cpp:5:15: error: 'value' is a private member of 'std::integral_constant<unsigned long, 2>'
    5 |     for (auto [name, value] : boost::process::environment::current())
      |               ^
/usr/include/boost/process/v2/environment.hpp:1280:82: note: constrained by implicitly private
      inheritance here
 1280 | class tuple_size<BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view> : integral_constant<std::size_t, 2u> {};
      |                                                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-suse-linux/15/../../../../include/c++/15/type_traits:94:28: note: 
      member is declared here
   94 |       static constexpr _Tp value = __v;
      |                            ^
1 error generated.