naive_solver regression due to `mk_evar` tactic in !289

Require Import stdpp.prelude.

Lemma silly (P : nat → Prop) (x : nat) :
  (∀ x', P x' → x' = 10) → P x → x + 1 = 11.
Proof. naive_solver. Qed.

Used to work, fails after !289 (merged).