Error C3646: Unknown override specifier

I teach C++ from the book, at the end of the chapter there are tasks and one of them is to make a function for counting characters in the Document class. When creating the class, a problem occurred: when starting the program, the error "C3646 begin: unknown definition specifier" appears. Same error with the end () function;

Document.h:

#include <list>
#include <vector>
#include <iostream>
using namespace std;

using Line = vector<char>;

struct Document {
    list<Line> line;
    Document() { line.push_back(Line{}); }

    Text_iterator begin() { return Text_iterator(line.begin(), line.begin()->begin()); } //здесь ошибка
    Text_iterator end() { return Text_iterator(line.end(), line.end()->end()); } //в этой строке так же

    int count();
};

istream& operator>> (istream& is, Document& d) {
    for (char ch; is.get(ch);) {
        d.line.back().push_back(ch);
        if (ch == '\n') d.line.push_back(Line{});

    }

    if (d.line.back().size()) d.line.push_back(Line{});

    return is;
}

class Text_iterator {
    list<Line>::iterator ln;
    Line::iterator pos;

public:
    Text_iterator (list<Line>::iterator ll, Line::iterator pp) : ln{ll}, pos{pp} {}
    char& operator* () { return *pos;  }

    Text_iterator& operator++ ();

    bool operator== (const Text_iterator& other) const { return ln == other.ln && pos == other.pos; }
    bool operator!= (const Text_iterator& other) const { return !(*this == other); }
};