def integral_closure(x):
    """
    Return the integral closure of ``x``.

    EXAMPLES::

        sage: integral_closure(QQ)
        Rational Field
        sage: K.<a> = QuadraticField(5)
        sage: O2 = K.order(2*a); O2
        Order in Number Field in a with defining polynomial x^2 - 5
        sage: integral_closure(O2)
        Maximal Order in Number Field in a with defining polynomial x^2 - 5
    """
    return x.integral_closure()