26 C.F.R. § 1.652(c)-2
If income is required to be distributed currently to a beneficiary, by a trust for a taxable year which does not end with or within the last taxable year of a beneficiary (because of the beneficiary's death), the extent to which the income is included in the gross income of the beneficiary for his last taxable year or in the gross income of his estate is determined by the computations under section 652 for the taxable year of the trust in which his last taxable year ends. Thus, the distributable net income of the taxable year of the trust determines the extent to which the income required to be distributed currently to the beneficiary is included in his gross income for his last taxable year or in the gross income of his estate. (Section 652(c) does not apply to such amounts.) The gross income for the last taxable year of a beneficiary on the cash basis includes only income actually distributed to the beneficiary before his death. Income required to be distributed, but in fact distributed to his estate, is included in the gross income of the estate as income in respect of a decedent under section 691. See paragraph (e) of § 1.663(c)-3 with respect to separate share treatment for the periods before and after the decedent's death. If the trust does not qualify as a simple trust for the taxable year of the trust in which the last taxable year of the beneficiary ends, see section 662(c) and § 1.662(c)-2 .
26 C.F.R. §1.652(c)-2