See https://www.law.cornell.edu/uscode/text/26/642
(i) In generalA qualified disability trust shall be allowed a deduction equal to the exemption amount under section 151(d), determined—
(I) by treating such trust as an individual described in section 68(b)(1)(C), and
(II)by applying section 67(e) (without the reference to section 642(b)) for purposes of determining the adjusted gross income of the trust.
(ii) Qualified disability trustFor purposes of clause (i), the term “qualified disability trust” means any trust if—
(II) all of the beneficiaries of the trust as of the close of the taxable year are determined by the Commissioner of Social Security to have been disabled (within the meaning of section 1614(a)(3) of the Social Security Act, 42 U.S.C. 1382c(a)(3)) for some portion of such year.
A trust shall not fail to meet the requirements of subclause (II) merely because the corpus of the trust may revert to a person who is not so disabled after the trust ceases to have any
beneficiary who is so disabled.
