Is a federal or state law error for failing to instruct on a requested affirmative defense instruction supported by substantial evidence?

Finally, we have yet to determine whether a trial courts failure to instruct on a requested affirmative defense instruction supported by substantial evidence is federal constitutional error or state law error. (See People v. Salas (2006) 37 Cal.4th 967, 984, 38 Cal.Rptr.3d 624, 127 P.3d 40 ( Salas ).) We need not decide that issue here. The duty to instruct on a defense such as self-defense or on accident³ only would have arisen if the

[418 P.3d 849]