theorem targetAffineLocallyOfOpenCover {P : AffineTargetMorphismProperty} (hP : P.IsLocal)
    {X Y : Scheme} (f : X ⟶ Y) (𝒰 : Y.OpenCover) [∀ i, IsAffine (𝒰.obj i)]
    (h𝒰 : ∀ i, P (pullback.snd : (𝒰.pullbackCover f).obj i ⟶ 𝒰.obj i)) :
    targetAffineLocally P f := by
  classical
  let S i := (⟨⟨Set.range (𝒰.map i).1.base, (𝒰.IsOpen i).base_open.open_range⟩,
    rangeIsAffineOpenOfOpenImmersion (𝒰.map i)⟩ : Y.affineOpens)
  intro U
  apply of_affine_open_cover (P := _) U (Set.range S)
  · intro U r h
    haveI : IsAffine _ := U.2
    have := hP.2 (f ∣_ U.1)
    replace this := this (Y.presheaf.map (eqToHom U.1.openEmbedding_obj_top).op r) h
    -- Porting note : the following 2 instances was not necessary
    haveI i1 : IsAffine (Y.restrict (Scheme.affineBasicOpen Y r).1.openEmbedding) :=
      (Scheme.affineBasicOpen Y r).2
    haveI i2 : IsAffine
      ((Y.restrict U.1.openEmbedding).restrict
        ((Y.restrict U.1.openEmbedding).basicOpen <|
          (Y.presheaf.map (eqToHom U.1.openEmbedding_obj_top).op r)).openEmbedding) :=
      IsAffineOpen.basicOpenIsAffine (X := Y.restrict U.1.openEmbedding) (topIsAffineOpen _) _
    rw [← P.toProperty_apply] at this ⊢
    exact (hP.1.arrow_mk_iso_iff (morphismRestrictRestrictBasicOpen f _ r)).mp this
  · intro U s hs H
    haveI : IsAffine _ := U.2
    apply hP.3 (f ∣_ U.1) (s.image (Y.presheaf.map (eqToHom U.1.openEmbedding_obj_top).op))
    · apply_fun Ideal.comap (Y.presheaf.map (eqToHom U.1.openEmbedding_obj_top.symm).op) at hs
      rw [Ideal.comap_top] at hs
      rw [← hs]
      simp only [eqToHom_op, eqToHom_map, Finset.coe_image]
      have : ∀ {R S : CommRingCat} (e : S = R) (s : Set S),
          Ideal.span (eqToHom e '' s) = Ideal.comap (eqToHom e.symm) (Ideal.span s) := by
        intro _ _ e _
        subst e
        simp only [eqToHom_refl, id_apply, Set.image_id']
        -- Porting note : Lean didn't see `𝟙 _` is just ring hom id
        exact (Ideal.comap_id _).symm
      apply this
    · rintro ⟨r, hr⟩
      obtain ⟨r, hr', rfl⟩ := Finset.mem_image.mp hr
      specialize H ⟨r, hr'⟩
      -- Porting note : the following 2 instances was not necessary
      haveI i1 : IsAffine (Y.restrict (Scheme.affineBasicOpen Y r).1.openEmbedding) :=
        (Scheme.affineBasicOpen Y r).2
      haveI i2 : IsAffine
        ((Y.restrict U.1.openEmbedding).restrict
          ((Y.restrict U.1.openEmbedding).basicOpen <|
            (Y.presheaf.map (eqToHom U.1.openEmbedding_obj_top).op r)).openEmbedding) :=
        IsAffineOpen.basicOpenIsAffine (X := Y.restrict U.1.openEmbedding) (topIsAffineOpen _) _
      rw [← P.toProperty_apply] at H ⊢
      exact (hP.1.arrow_mk_iso_iff (morphismRestrictRestrictBasicOpen f _ r)).mpr H
  · rw [Set.eq_univ_iff_forall]
    simp only [Set.mem_iUnion]
    intro x
    

Proof state:
case hS
P : AffineTargetMorphismProperty
hP : AffineTargetMorphismProperty.IsLocal P
X Y : Scheme
f : X ⟶ Y
𝒰 : Scheme.OpenCover Y
inst✝ : ∀ (i : 𝒰.J), IsAffine (Scheme.OpenCover.obj 𝒰 i)
h𝒰 : ∀ (i : (Scheme.OpenCover.pullbackCover 𝒰 f).J), P pullback.snd
S : 𝒰.J → { x // x ∈ Scheme.affineOpens Y } :=
  fun i =>
    {
      val :=
        { carrier := Set.range ⇑(Scheme.OpenCover.map 𝒰 i).val.base,
          is_open' := (_ : IsOpen (Set.range ⇑(Scheme.OpenCover.map 𝒰 i).val.base)) },
      property := (_ : IsAffineOpen (Scheme.Hom.opensRange (Scheme.OpenCover.map 𝒰 i))) }
U : ↑(Scheme.affineOpens Y)
x : ↑↑Y.toPresheafedSpace
⊢ ∃ i, x ∈ ↑↑↑i

Next tactic:
exact ⟨⟨_, ⟨𝒰.f x, rfl⟩⟩, 𝒰.Covers x⟩