From stdpp Require Import tactics.

Goal ∀ P1 P2 P3 P4 (P: Prop),
  P1 ∨ (Is_true (P2 || P3)) ∨ P4 →
  (P1 → P) →
  (P2 → P) →
  (P3 → P) →
  (P4 → P) →
  P.
Proof.
  intros * HH X1 X2 X3 X4.
  destruct_or? HH; [ exact (X1 HH) | exact (X2 HH) | exact (X3 HH) | exact (X4 HH) ].
Qed.

Goal ∀ P1 P2 P3 P4 (P: Prop),
  P1 ∨ P2 →
  P3 ∨ P4 →
  (P1 → P3 → P) →
  (P1 → P4 → P) →
  (P2 → P3 → P) →
  (P2 → P4 → P) →
  P.
Proof.
  intros * HH1 HH2 X1 X2 X3 X4.
  destruct_or?; [ exact (X1 HH1 HH2) | exact (X3 HH1 HH2) | exact (X2 HH1 HH2) | exact (X4 HH1 HH2) ].
Qed.

Goal ∀ P1 P2 P3 P4 (P: Prop),
  id (P1 ∨ P2) →
  id (P3 ∨ P4) →
  (P1 → P3 → P) →
  (P1 → P4 → P) →
  (P2 → P3 → P) →
  (P2 → P4 → P) →
  P.
Proof.
  intros * HH1 HH2 X1 X2 X3 X4.
  Fail progress destruct_or?.
  Fail progress destruct_or!.
  destruct_or! HH1; destruct_or! HH2;
  [ exact (X1 HH1 HH2) | exact (X2 HH1 HH2) | exact (X3 HH1 HH2) | exact (X4 HH1 HH2) ].
Qed.

Goal ∀ P1 P2 P3 P4,
  P1 ∧ (Is_true (P2 && P3)) ∧ P4 →
  P1 ∧ P2 ∧ P3.
Proof.
  intros * HH. split_and!; [ destruct_and? HH; assumption | destruct_and?; assumption | ].
  destruct_and?. Fail destruct_and!. assumption.
Qed.

Goal ∀ (n : nat), ∃ m : nat, True.
Proof. intros ?. rename select nat into m. exists m. done. Qed.

Goal ∀ (P : nat → Prop), P 3 → P 4 → P 4.
Proof. intros P **. rename select (P _) into HP4. apply HP4. Qed.

(** [mk_evar] works on things that coerce to types. *)
(** This is a feature when we have packed structures, for example Iris's [ofe]
(fields other than the carrier omitted). *)
Structure ofe := Ofe { ofe_car :> Type }.
Goal ∀ A : ofe, True.
intros A.
let x := mk_evar A in idtac.
Abort.

(** More surprisingly, it also works for other coercions into a
universe, like [Is_true : bool → Prop]. *)
Goal True.
let x := mk_evar true in idtac.
Abort.