Generic repository filter data of T

I have a generic repository and i want to filter the data on Get in the generic repository. This method get the data and then other get methods call and return data

private IQueryable<T> fetchData(Expression<Func<T, bool>> filter)
{
    IQueryable<T> query = dbSet;

    if (filter != null) query = query.Where(filter);
}

This is the DbSet

private readonly IDbSet<T> dbSet;

and the class implemention is

public class Repository<T> : IRepository<T> where T : class

These are the classes and interfaces on Model class

public interface IEntity<T>
{
    T Id { get; set; }
}
public interface IAuditableEntity
{
    bool IsDeleted { get; set; }
}
public abstract class Entity<T> : IEntity<T>
{
    [Key]
    public virtual T Id { get; set; }
}
public abstract class AuditableEntity : Entity<string>, IEntity<string>, IAuditableEntity
{
    public bool IsDeleted { get; set; }

}
[Table("Contact")]
public class Contact : AuditableEntity
{
    [Required]
    [StringLength(250)]
    public string FirstName { get; set; }

    [StringLength(250)]
    public string MiddleName { get; set; }

    [Required]
    [StringLength(250)]
    public string LastName { get; set; }
}

Now i want to filter the data in fetchData method so that only those records return which are deleted == false as this is a common functionality and should be implemented in repository base level not on individual repository or business logic classes.

The problem is that once i cast IQueryable<T> query to IQueryable<IAuditableEntity> query i cannot convert it back to IQueryable or for further filteration of data and return. I tried these but didn't work

var dbSetIAuditableEntity = dbSet as IQueryable<AuditableEntity>;

if (dbSetIAuditableEntity != null)
{
    dbSetIAuditableEntity = dbSetIAuditableEntity.Where(d => d.IsDeleted == false);
    query = (IQueryable<T>)((object)dbSetIAuditableEntity);

    query = dbSet.Where(d => ((IAuditableEntity)d).IsDeleted == false);
}

Any suggestions to implement this functionality is generic repository

Edit

Actually all entities don't implement AuditableEntity some implement only IEntity<String> OR IEntity<int> and some implement nothing. It's not very good approach but this is the situation.

Edit 2

This is how i instantiate Generic repository By Autofac

builder.RegisterGeneric(typeof(Repository<>)).As(typeof(IRepository<>));