/-- The unique uniform structure inducing a given compact topological structure. -/
def uniformSpaceOfCompactT2 [TopologicalSpace γ] [CompactSpace γ] [T2Space γ] : UniformSpace γ
    where
  uniformity := 𝓝ˢ (diagonal γ)
  refl := principal_le_nhdsSet
  symm := continuous_swap.tendsto_nhdsSet fun x => Eq.symm
  comp := by
    /-  This is the difficult part of the proof. We need to prove that, for each neighborhood `W`
        of the diagonal `Δ`, there exists a smaller neighborhood `V` such that `V ○ V ⊆ W`.
        -/
    set 𝓝Δ := 𝓝ˢ (diagonal γ)
    -- The filter of neighborhoods of Δ
    set F := 𝓝Δ.lift' fun s : Set (γ × γ) => s ○ s
    -- Compositions of neighborhoods of Δ
    -- If this weren't true, then there would be V ∈ 𝓝Δ such that F ⊓ 𝓟 Vᶜ ≠ ⊥
    rw [le_iff_forall_inf_principal_compl]
    intro V V_in
    by_contra H
    haveI : NeBot (F ⊓ 𝓟 Vᶜ) := ⟨H⟩
    -- Hence compactness would give us a cluster point (x, y) for F ⊓ 𝓟 Vᶜ
    obtain ⟨⟨x, y⟩, hxy⟩ : ∃ p : γ × γ, ClusterPt p (F ⊓ 𝓟 Vᶜ) := cluster_point_of_compact _
    -- In particular (x, y) is a cluster point of 𝓟 Vᶜ, hence is not in the interior of V,
    -- and a fortiori not in Δ, so x ≠ y
    have clV : ClusterPt (x, y) (𝓟 <| Vᶜ) := hxy.of_inf_right
    have : (x, y) ∉ interior V := by
      have : (x, y) ∈ closure Vᶜ := by rwa [mem_closure_iff_clusterPt]
      rwa [closure_compl] at this
    have diag_subset : diagonal γ ⊆ interior V := subset_interior_iff_mem_nhdsSet.2 V_in
    have x_ne_y : x ≠ y := mt (@diag_subset (x, y)) this
    -- Since γ is compact and Hausdorff, it is T₄, hence T₃.
    -- So there are closed neighborhoods V₁ and V₂ of x and y contained in
    -- disjoint open neighborhoods U₁ and U₂.
    obtain
      ⟨U₁, _, V₁, V₁_in, U₂, _, V₂, V₂_in, V₁_cl, V₂_cl, U₁_op, U₂_op, VU₁, VU₂, hU₁₂⟩ :=
      disjoint_nested_nhds x_ne_y
    -- We set U₃ := (V₁ ∪ V₂)ᶜ so that W := U₁ ×ˢ U₁ ∪ U₂ ×ˢ U₂ ∪ U₃ ×ˢ U₃ is an open
    -- neighborhood of Δ.
    let U₃ := (V₁ ∪ V₂)ᶜ
    have U₃_op : IsOpen U₃ := (V₁_cl.union V₂_cl).isOpen_compl
    let W := U₁ ×ˢ U₁ ∪ U₂ ×ˢ U₂ ∪ U₃ ×ˢ U₃
    have W_in : W ∈ 𝓝Δ := by
      rw [mem_nhdsSet_iff_forall]
      rintro ⟨z, z'⟩ (rfl : z = z')
      refine' IsOpen.mem_nhds _ _
      · apply_rules [IsOpen.union, IsOpen.prod]
      · simp only [mem_union, mem_prod, and_self_iff]
        exact (_root_.em _).imp_left fun h => union_subset_union VU₁ VU₂ h
    -- So W ○ W ∈ F by definition of F
    have : W ○ W ∈ F := @mem_lift' _ _ _ (fun s => s ○ s) _ W_in
      -- Porting note: was `by simpa only using mem_lift' W_in`
    -- And V₁ ×ˢ V₂ ∈ 𝓝 (x, y)
    

Proof state:
case intro.mk.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
α : Type u_1
β : Type u_2
γ : Type u_3
inst✝⁴ : UniformSpace α
inst✝³ : UniformSpace β
inst✝² : TopologicalSpace γ
inst✝¹ : CompactSpace γ
inst✝ : T2Space γ
𝓝Δ : Filter (γ × γ) := 𝓝ˢ (diagonal γ)
F : Filter (γ × γ) := Filter.lift' 𝓝Δ fun s => s ○ s
V : Set (γ × γ)
V_in : V ∈ 𝓝Δ
H : ¬F ⊓ 𝓟 Vᶜ = ⊥
this✝¹ : NeBot (F ⊓ 𝓟 Vᶜ)
x y : γ
hxy : ClusterPt (x, y) (F ⊓ 𝓟 Vᶜ)
clV : ClusterPt (x, y) (𝓟 Vᶜ)
this✝ : (x, y) ∉ interior V
diag_subset : diagonal γ ⊆ interior V
x_ne_y : x ≠ y
U₁ : Set γ
left✝¹ : U₁ ∈ 𝓝 x
V₁ : Set γ
V₁_in : V₁ ∈ 𝓝 x
U₂ : Set γ
left✝ : U₂ ∈ 𝓝 y
V₂ : Set γ
V₂_in : V₂ ∈ 𝓝 y
V₁_cl : IsClosed V₁
V₂_cl : IsClosed V₂
U₁_op : IsOpen U₁
U₂_op : IsOpen U₂
VU₁ : V₁ ⊆ U₁
VU₂ : V₂ ⊆ U₂
hU₁₂ : Disjoint U₁ U₂
U₃ : Set γ := (V₁ ∪ V₂)ᶜ
U₃_op : IsOpen U₃
W : Set (γ × γ) := U₁ ×ˢ U₁ ∪ U₂ ×ˢ U₂ ∪ U₃ ×ˢ U₃
W_in : W ∈ 𝓝Δ
this : W ○ W ∈ F
⊢ False

Next tactic:
have hV₁₂ : V₁ ×ˢ V₂ ∈ 𝓝 (x, y) := prod_mem_nhds V₁_in V₂_in