((a)) Except as provided in subsection (b), a reserve commissioned officer who has not completed the period of service required of him by or any other provision of law may not be discharged or transferred from an active status under chapter 843, 1407, 1409, or 1411 of this title or chapter 21 of title 14. Unless, under regulations prescribed by the Secretary concerned, he is promoted to a higher reserve grade, he shall be retained in an active status in his reserve grade for the rest of his period of required service and shall be an additional number to the authorized strength of his grade.
((b)) Subsection (a) does not prevent the discharge or transfer from an active status of—
((1)) a commissioned warrant officer;
((2)) an officer on the active-duty list or a reserve active-status list who is found not qualified for promotion to the grade of first lieutenant, in the case of an officer of the Army, Air Force, or Marine Corps, or lieutenant (junior grade), in the case of an officer of the Navy;
((3)) an officer on the active-duty list or reserve active-status list who has failed of selection for promotion for the second time to the grade of captain, in the case of an officer of the Army, Air Force, or Marine Corps, or to the grade of lieutenant, in the case of an officer of the Navy; or
((4)) an officer whose discharge or transfer from an active status is required by law.