运算符的要求<恒定性在标准：：stable_sort

我对 std：：sort 和 std：：stable_sortoperator<const限定符的要求差异有点困惑。假设一个简单的结构：

#include <vector>
#include <algorithm>
struct Custom {
bool operator<(const Custom& custom) /* const */{
return true;
};
};

如果我们尝试编译并运行此代码，一切正常：

int main() {
std::vector<Custom> values(3);
std::sort(values.begin(), values.end());
return 0;
}

但是这段带有std::stable_sort的代码编译失败：

int main() {
std::vector<Custom> values(3);
std::sort(values.begin(), values.end());
return 0;
}

下面是错误堆栈跟踪：

In file included from /usr/include/c++/5/bits/stl_algobase.h:71:0,
from /usr/include/c++/5/bits/char_traits.h:39,
from /usr/include/c++/5/ios:40,
from /usr/include/c++/5/ostream:38,
from /usr/include/c++/5/iostream:39,
from temp.cpp:1:
/usr/include/c++/5/bits/predefined_ops.h: In instantiation of ‘bool __gnu_cxx::__ops::_Val_less_iter::operator()(_Value&, _Iterator) const [with _Value = const Custom; _Iterator = __gnu_cxx::__normal_iterator<Custom*, std::vector<Custom> >]’:
/usr/include/c++/5/bits/stl_algo.h:2050:14:   required from ‘_ForwardIterator std::__upper_bound(_ForwardIterator, _ForwardIterator, const _Tp&, _Compare) [with _ForwardIterator = __gnu_cxx::__normal_iterator<Custom*, std::vector<Custom> >; _Tp = Custom; _Compare = __gnu_cxx::__ops::_Val_less_iter]’
/usr/include/c++/5/bits/stl_algo.h:2522:26:   required from ‘void std::__merge_without_buffer(_BidirectionalIterator, _BidirectionalIterator, _BidirectionalIterator, _Distance, _Distance, _Compare) [with _BidirectionalIterator = __gnu_cxx::__normal_iterator<Custom*, std::vector<Custom> >; _Distance = long int; _Compare = __gnu_cxx::__ops::_Iter_less_iter]’
/usr/include/c++/5/bits/stl_algo.h:2782:34:   required from ‘void std::__inplace_stable_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Custom*, std::vector<Custom> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]’
/usr/include/c++/5/bits/stl_algo.h:4863:28:   required from ‘void std::__stable_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Custom*, std::vector<Custom> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]’
/usr/include/c++/5/bits/stl_algo.h:4897:36:   required from ‘void std::stable_sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<Custom*, std::vector<Custom> >]’
temp.cpp:15:45:   required from here
/usr/include/c++/5/bits/predefined_ops.h:71:22: error: no match for ‘operator<’ (operand types are ‘const Custom’ and ‘Custom’)
{ return __val < *__it; }

所以我的问题是： 这仅仅是技术实现细节的结果，还是存在一些支持这种行为的客观论据？