I want to get the type of the iterator to the const-casted contents of the vector. I thought I can use decltype for it.

Apparently it is not that simple.

#include <ranges>
#include <vector>

const int& cast_const(int& i)  {
   return i;
}

struct PoC {
   std::vector<int> _m;
   decltype(_m | std::views::transform(cast_const)) _const = _m | std::views::transform(cast_const);
   using const_iterator = decltype(_const.begin());
   [[nodiscard]] auto cbegin() const { return _const.begin(); }
   using iterator = decltype(_m.begin());
   [[nodiscard]] auto begin() { return _m.begin(); }
};

void test() {
   PoC p;
   PoC::const_iterator cit = p.cbegin(); // ERROR: error: conversion from ‘_Iterator<true>’ to non-scalar type ‘_Iterator<false>’ requested
   PoC::iterator it = p.begin(); // Works, because no ranges.
}

I believe it boils down to the fact, that PoC::const_iterator is of type std::ranges::transform_view<std::ranges::ref_view<std::vector<int> >, int& (*)(int&)>::_Iterator<false>, while the return value of p.cbegin() is of type std::ranges::transform_view<std::ranges::ref_view<std::vector<int> >, int& (*)(int&)>::_Iterator<true>.

The argument type of the _Iterator is named _Const in the source code for ranges.

Can you explain me the reason why the ranges are so designed? Is the problem fixable, or should I build the const_iterator from scratch, and not use the ranges?