Method underflow

src/backward.h:3936–3936

public:
    cfile_streambuf(FILE* _sink) : sink(_sink) {}
    int_type underflow() override { return traits_type::eof(); }
    int_type overflow(int_type ch) override
    {
        if (traits_type::not_eof(ch) && fputc(ch, sink) != EOF) {