Несоответствие в поиске имени среди разных компиляторов

Прежде всего, не стесняйтесь предлагать лучшее название для этого вопроса.

Рассмотрим следующую программу:

#include <numeric>
namespace N { class C {}; }
int operator+( int i, N::C ) { return i+1; }
int main() {
     N::C a[10];
     std::accumulate( a, a+10, 0 );
} 

g++ 5.4.0: компиляция прошла успешно (см. живую демонстрацию здесь)

clang++ 3.8.0 (см. живую демонстрацию здесь)

Ошибки:

In file included from source_file.cpp:3:
/usr/include/c++/v1/numeric:75:25: error: invalid operands to binary expression ('int' and 'N::C')
        __init = __init + *__first;
                 ~~~~~~ ^ ~~~~~~~~
source_file.cpp:8:11: note: in instantiation of function template specialization 'std::__1::accumulate<N::C *, int>' requested here
     std::accumulate( a, a+10, 0 );
          ^
/usr/include/c++/v1/iterator:640:1: note: candidate template ignored: could not match 'reverse_iterator<type-parameter-0-0>' against 'N::C'
operator+(typename reverse_iterator<_Iter>::difference_type __n, const reverse_iterator<_Iter>& __x)
^
/usr/include/c++/v1/iterator:1044:1: note: candidate template ignored: could not match 'move_iterator<type-parameter-0-0>' against 'N::C'
operator+(typename move_iterator<_Iter>::difference_type __n, const move_iterator<_Iter>& __x)
^
/usr/include/c++/v1/iterator:1400:1: note: candidate template ignored: could not match '__wrap_iter<type-parameter-0-0>' against 'N::C'
operator+(typename __wrap_iter<_Iter>::difference_type __n,
^
1 error generated.

Microsoft Visual C++ 19.00.23506 (см. живую демонстрацию здесь)

Ошибки:

C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\numeric(20): error C2672: 'operator __surrogate_func': no matching overloaded function found
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\numeric(30): note: see reference to function template instantiation '_Ty std::_Accumulate<_Iter,_Ty,_Fn2>(_InIt,_InIt,_Ty,_Fn2)' being compiled
        with
        [
            _Ty=int,
            _Iter=N::C *,
            _Fn2=std::plus<void>,
            _InIt=N::C *
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\numeric(38): note: see reference to function template instantiation '_Ty std::accumulate<_InIt,_Ty,std::plus<void>>(_InIt,_InIt,_Ty,_Fn2)' being compiled
        with
        [
            _Ty=int,
            _InIt=N::C *,
            _Fn2=std::plus<void>
        ]
source_file.cpp(8): note: see reference to function template instantiation '_Ty std::accumulate<N::C*,int>(_InIt,_InIt,_Ty)' being compiled
        with
        [
            _Ty=int,
            _InIt=N::C *
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\numeric(20): error C2893: Failed to specialize function template 'unknown-type std::plus<void>::operator ()(_Ty1 &&,_Ty2 &&) const'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\numeric(20): note: With the following template arguments:
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\numeric(20): note: '_Ty1=int &'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\numeric(20): note: '_Ty2=N::C &'

    Error(s):
    In file included from source_file.cpp:3:
    /usr/include/c++/v1/numeric:75:25: error: invalid operands to binary expression ('int' and 'N::C')
            __init = __init + *__first;
                     ~~~~~~ ^ ~~~~~~~~
    source_file.cpp:8:11: note: in instantiation of function template specialization 'std::__1::accumulate<N::C *, int>' requested here
         std::accumulate( a, a+10, 0 );
              ^
    /usr/include/c++/v1/iterator:640:1: note: candidate template ignored: could not match 'reverse_iterator<type-parameter-0-0>' against 'N::C'
    operator+(typename reverse_iterator<_Iter>::difference_type __n, const reverse_iterator<_Iter>& __x)
    ^
    /usr/include/c++/v1/iterator:1044:1: note: candidate template ignored: could not match 'move_iterator<type-parameter-0-0>' against 'N::C'
    operator+(typename move_iterator<_Iter>::difference_type __n, const move_iterator<_Iter>& __x)
    ^
    /usr/include/c++/v1/iterator:1400:1: note: candidate template ignored: could not match '__wrap_iter<type-parameter-0-0>' against 'N::C'
    operator+(typename __wrap_iter<_Iter>::difference_type __n,
    ^
    1 error generated.

Эта программа на удивление компилируется без ошибок и на компиляторе Intel C++.

Итак, вопрос в том, какие компиляторы здесь правильные? Этот код плохо сформирован? Что стандарт говорит об этом?