Use `TCEq` so that TC search can establish non-valueness.

theories/program_logic/weakestpre.v

@@ -79,8 +79,8 @@ Global Instance wp_proper s E e :
 Proof.
   by intros Φ Φ' ?; apply equiv_dist=>n; apply wp_ne=>v; apply equiv_dist.
 Qed.
-Lemma wp_contractive s E e n :
-  to_val e = None →
+Global Instance wp_contractive s E e n :
+  TCEq (to_val e) None →
   Proper (pointwise_relation _ (dist_later n) ==> dist n) (wp (PROP:=iProp Σ) s E e).
 Proof.
   intros He Φ Ψ HΦ. rewrite !wp_unfold /wp_pre He.