Require Import Coq.Classes.Equivalence.
Require Import Coq.Program.Program.
Import Relation_Definitions.
Import Morphisms.

Obligation Tactic := program_simpl ; simpl_relation.
Generalizable Variables A eqA.

Lemma bla : forall `{ ! @Equivalence A (eqA : relation A) } x y, eqA x y -> eqA y x.
Proof.
  intros.
  rewrite H0.
  reflexivity.
Defined.

Lemma bla' : forall `{ ! @Equivalence A (eqA : relation A) } x y, eqA x y -> eqA y x.
Proof.
  intros.
  (* Need delta on [relation] to unify with the right lemmas. *)
  rewrite <- H0.
  reflexivity.
Qed.

Axiom euclid : nat -> { x : nat | x > 0 } -> nat.

Definition eq_proj {A} {s : A -> Prop} : relation (sig s) :=
  fun x y => `x = `y.

#[export] Program Instance foo {A : Type} {s : A -> Prop} : @Equivalence (sig s) eq_proj.

Next Obligation.
Proof.
  cbv in *;congruence.
Qed.

#[export] Instance bar : Proper (eq ==> eq_proj ==> eq) euclid.
Proof.
Admitted.

Goal forall (x : nat) (y : nat | y > 0) (z : nat | z > 0), eq_proj y z -> euclid x y = euclid x z.
Proof.
  intros.
  (* Breaks if too much delta in unification *)
  rewrite H.
  reflexivity.
Qed.
