What is the legal test for a search incident to arrest?

Conditioning a search incident to arrest on the degree to which an otherwise constitutional arrest complies with state procedure, moreover, makes no sense. The justification for a search incident to arrest rests on "(1) the need to disarm the suspect in order to take him into custody, and (2) the need to preserve evidence for later use at trial." (Knowles v. Iowa, supra, 525 U.S. at p. 116, 119 S.Ct. 484.) Neither the need to disarm the suspect nor the need to preserve evidence depends upon the arrest's compliance with state law.