/-- Elements of the uniformity (defined generally for completions) can be characterized in terms
of the distance. -/
protected theorem mem_uniformity_dist (s : Set (Completion α × Completion α)) :
    s ∈ 𝓤 (Completion α) ↔ ∃ ε > 0, ∀ {a b}, dist a b < ε → (a, b) ∈ s := by
  constructor
  · /- Start from an entourage `s`. It contains a closed entourage `t`. Its pullback in `α` is an
      entourage, so it contains an `ε`-neighborhood of the diagonal by definition of the entourages
      in metric spaces. Then `t` contains an `ε`-neighborhood of the diagonal in `Completion α`, as
      closed properties pass to the completion. -/
    intro hs
    rcases mem_uniformity_isClosed hs with ⟨t, ht, ⟨tclosed, ts⟩⟩
    have A : { x : α × α | (↑x.1, ↑x.2) ∈ t } ∈ uniformity α :=
      uniformContinuous_def.1 (uniformContinuous_coe α) t ht
    rcases mem_uniformity_dist.1 A with ⟨ε, εpos, hε⟩
    refine' ⟨ε, εpos, @fun x y hxy ↦ _⟩
    have : ε ≤ dist x y ∨ (x, y) ∈ t := by
      refine' induction_on₂ x y _ _
      · have : { x : Completion α × Completion α | ε ≤ dist x.fst x.snd ∨ (x.fst, x.snd) ∈ t } =
               { p : Completion α × Completion α | ε ≤ dist p.1 p.2 } ∪ t := by ext; simp
        rw [this]
        apply IsClosed.union _ tclosed
        exact isClosed_le continuous_const Completion.uniformContinuous_dist.continuous
      · intro x y
        rw [Completion.dist_eq]
        by_cases h : ε ≤ dist x y
        · exact Or.inl h
        · have Z := hε (not_le.1 h)
          simp only [Set.mem_setOf_eq] at Z
          exact Or.inr Z
    simp only [not_le.mpr hxy, false_or_iff, not_le] at this
    exact ts this
  · /- Start from a set `s` containing an ε-neighborhood of the diagonal in `Completion α`. To show
        that it is an entourage, we use the fact that `dist` is uniformly continuous on
        `Completion α × Completion α` (this is a general property of the extension of uniformly
        continuous functions). Therefore, the preimage of the ε-neighborhood of the diagonal in ℝ
        is an entourage in `Completion α × Completion α`. Massaging this property, it follows that
        the ε-neighborhood of the diagonal is an entourage in `Completion α`, and therefore this is
        also the case of `s`. -/
    rintro ⟨ε, εpos, hε⟩
    let r : Set (ℝ × ℝ) := { p | dist p.1 p.2 < ε }
    have : r ∈ uniformity ℝ := Metric.dist_mem_uniformity εpos
    have T := uniformContinuous_def.1 (@Completion.uniformContinuous_dist α _) r this
    simp only [uniformity_prod_eq_prod, mem_prod_iff, exists_prop, Filter.mem_map,
      Set.mem_setOf_eq] at T
    rcases T with ⟨t1, ht1, t2, ht2, ht⟩
    refine' mem_of_superset ht1 _
    have A : ∀ a b : Completion α, (a, b) ∈ t1 → dist a b < ε := by
      intro a b hab
      have : ((a, b), (a, a)) ∈ t1 ×ˢ t2 := ⟨hab, refl_mem_uniformity ht2⟩
      have I := ht this
      

Proof state:
α : Type u
β : Type v
inst✝ : PseudoMetricSpace α
s : Set (Completion α × Completion α)
ε : ℝ
εpos : ε > 0
hε : ∀ {a b : Completion α}, dist a b < ε → (a, b) ∈ s
r : Set (ℝ × ℝ) := {p | dist p.1 p.2 < ε}
this✝ : r ∈ 𝓤 ℝ
t1 : Set (Completion α × Completion α)
ht1 : t1 ∈ 𝓤 (Completion α)
t2 : Set (Completion α × Completion α)
ht2 : t2 ∈ 𝓤 (Completion α)
ht : t1 ×ˢ t2 ⊆ (fun p => ((p.1.1, p.2.1), p.1.2, p.2.2)) ⁻¹' {x | dist (dist x.1.1 x.1.2) (dist x.2.1 x.2.2) < ε}
a b : Completion α
hab : (a, b) ∈ t1
this : ((a, b), a, a) ∈ t1 ×ˢ t2
I : ((a, b), a, a) ∈ (fun p => ((p.1.1, p.2.1), p.1.2, p.2.2)) ⁻¹' {x | dist (dist x.1.1 x.1.2) (dist x.2.1 x.2.2) < ε}
⊢ dist a b < ε

Next tactic:
simp? [Completion.dist_self, Real.dist_eq, Completion.dist_comm] at I says
        simp only [Real.dist_eq, preimage_setOf_eq, mem_setOf_eq, Completion.dist_self,
          Completion.dist_comm, zero_sub, abs_neg] at I