// b.r < b.w => buffer is not empty
		m, err := b.writeBuf(w)
		n += m
		if err != nil {
			return n, err
		}
		b.fill() // buffer is empty
	}

	if b.err == io.EOF {
		b.err = nil
	}

	return n, b.readErr()
}

var errNegativeWrite = errors.New("bufio: writer returned negative count from Write")

// writeBuf writes the [Reader]'s buffer to the writer.