There is included in the gross income of a beneficiary of an estate or complex trust the sum of:

(a) Amounts of income required to be distributed currently to him, and

(b) All other amounts properly paid, credited, or required to be distributed to him

by the estate or trust. The preceding sentence is subject to the rules contained in §1.662(a)-2 (relating to currently distributable income), §1.662(a)-3 (relating to other amounts distributed), and §§1.662(b)-1 and 1.662(b)-2 (relating to character of amounts). Section 662 has no application to amounts excluded under section 663(a).