Hint Extern 2 (Reflexive (_ + _)) =>
eapply sum_rel_refl : typeclass_instances.

(* Specifications: *)
Lemma sum_rel_corefl {A B} (R1: rel A A) (R2: rel B B):
Coreflexive R1 -> Coreflexive R2 -> Coreflexive (R1 + R2).