该函数声明为constexpr，std::distance的重载为constexpr。但是当我用gcc7.2编译时，我得到一个我不理解的错误。
在我看来，constexpr std::distance中存在一个错误。
该错误表示它调用的std::__iterator_category(__first));不是constexpr。
这是代码和错误
#include <iostream>
#include <algorithm>
#include <iterator>
#include <stdexcept>
#include <array>
#include <string>
#include <cassert>
template<typename It>
constexpr auto max (It first, It last)
{
auto count = std::distance(first, last);
if (count == 0)
throw std::domain_error{"max is undefined on empty range"};
if (count == 1) return *first;
auto mid = first + (last - first) / 2;
return std::max(max(first, mid), max(mid, last));
}
int main()
{
constexpr std::array<int, 4> vs{1,6,8,3};
constexpr auto max_of_vs = max(std::cbegin(vs), std::cend(vs));
assert(max_of_vs == 8);
std::cout << max_of_vs << std::endl;
std::array<int, 5> xs{1,9,8,3,6};
auto max_of_xs = max(std::begin(xs), std::end(xs));
assert(max_of_xs == 9);
std::cout << max_of_xs << std::endl;
//std::array<int, 0> zs{};
//auto max_of_zs = max(std::begin(zs), std::end(zs)); // throws at runtime
//assert(max_of_zs == 0);
}
错误：
main.cpp: In function 'int main()':
main.cpp:23:33: in constexpr expansion of 'max<const int*>(std::cbegin<std::array<int, 4> >(vs), std::cend<std::array<int, 4> >(vs))'
main.cpp:12:29: error: 'constexpr typename std::iterator_traits<_Iterator>::difference_type std::distance(_InputIterator, _InputIterator) [with _InputIterator = const int*; typename std::iterator_traits<_Iterator>::difference_type = long int]' called in a constant expression
auto count = std::distance(first, last);
~~~~~~~~~~~~~^~~~~~~~~~~~~
In file included from /usr/local/include/c++/7.2.0/bits/stl_algobase.h:66:0,
from /usr/local/include/c++/7.2.0/bits/char_traits.h:39,
from /usr/local/include/c++/7.2.0/ios:40,
from /usr/local/include/c++/7.2.0/ostream:38,
from /usr/local/include/c++/7.2.0/iostream:39,
from main.cpp:1:
/usr/local/include/c++/7.2.0/bits/stl_iterator_base_funcs.h:138:5: note: 'constexpr typename std::iterator_traits<_Iterator>::difference_type std::distance(_InputIterator, _InputIterator) [with _InputIterator = const int*; typename std::iterator_traits<_Iterator>::difference_type = long int]' is not usable as a constexpr function because:
distance(_InputIterator __first, _InputIterator __last)
^~~~~~~~
/usr/local/include/c++/7.2.0/bits/stl_iterator_base_funcs.h:142:33: error: call to non-constexpr function 'typename std::iterator_traits<_Iterator>::iterator_category std::__iterator_category(const _Iter&) [with _Iter = const int*; typename std::iterator_traits<_Iterator>::iterator_category = std::random_access_iterator_tag]'
std::__iterator_category(__first));
~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~