Module: Mathlib.Topology.Instances.Complex

Goal state before:
pp: K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
⊢ (↑↑ψ).toFun = ⇑K.subtype ∨ (↑↑ψ).toFun = ⇑(starRingEnd ℂ) ∘ ⇑K.subtype

Tactic:
let : IsTopologicalDivisionRing ℂ := IsTopologicalDivisionRing.mk
  let : IsTopologicalRing K.topologicalClosure :=
    Subring.instIsTopologicalRing K.topologicalClosure.toSubring
  set ι : K → K.topologicalClosure := ⇑(Subfield.inclusion K.le_topologicalClosure)
  have ui : IsUniformInducing ι :=
    ⟨by
      rw [uniformity_subtype, uniformity_subtype, Filter.comap_comap]
      congr ⟩
  let di := ui.isDenseInducing (?_ : DenseRange ι)
  · -- extψ : closure(K) →+* ℂ is the extension of ψ : K →+* ℂ
    let extψ := IsDenseInducing.extendRingHom ui di.dense hc
    have hψ := (uniformContinuous_uniformly_extend ui di.dense hc).continuous
    rcases Complex.subfield_eq_of_closed (Subfield.isClosed_topologicalClosure K) with h | h
    · left
      let j := RingEquiv.subfieldCongr h
      -- ψ₁ is the continuous ring hom `ℝ →+* ℂ` constructed from `j : closure (K) ≃+* ℝ`
      -- and `extψ : closure (K) →+* ℂ`
      let ψ₁ := RingHom.comp extψ (RingHom.comp j.symm.toRingHom ofRealHom.rangeRestrictField)
      -- Porting note: was `by continuity!` and was used inline
      have hψ₁ : Continuous ψ₁ := by
        simpa only [RingHom.coe_comp] using! hψ.comp ((continuous_algebraMap ℝ ℂ).subtype_mk _)
      ext1 x
      rsuffices ⟨r, hr⟩ : ∃ r : ℝ, ofRealHom.rangeRestrictField r = j (ι x)
      · have := RingHom.congr_fun (ringHom_eq_ofReal_of_continuous hψ₁) r
        rw [RingHom.comp_apply, RingHom.comp_apply, hr, RingEquiv.toRingHom_eq_coe] at this
        convert! this using 1
        · exact (IsDenseInducing.extend_eq di hc.continuous _).symm
        · rw [← ofRealHom.coe_rangeRestrictField, hr]
          rfl
      obtain ⟨r, hr⟩ := SetLike.coe_mem (j (ι x))
      exact ⟨r, Subtype.ext hr⟩
    · -- ψ₁ is the continuous ring hom `ℂ →+* ℂ` constructed from `closure (K) ≃+* ℂ`
      -- and `extψ : closure (K) →+* ℂ`
      let ψ₁ :=
        RingHom.comp extψ
          (RingHom.comp (RingEquiv.subfieldCongr h).symm.toRingHom
            (@Subfield.topEquiv ℂ _).symm.toRingHom)
      -- Porting note: was `by continuity!` and was used inline
      have hψ₁ : Continuous ψ₁ := by
        simpa only [RingHom.coe_comp] using! hψ.comp (continuous_id.subtype_mk _)
      rcases ringHom_eq_id_or_conj_of_continuous hψ₁ with h | h
      · left
        ext1 z
        convert! RingHom.congr_fun h z using 1
        exact (IsDenseInducing.extend_eq di hc.continuous z).symm
      · right
        ext1 z
        convert! RingHom.congr_fun h z using 1
        exact (IsDenseInducing.extend_eq di hc.continuous z).symm
  · let j : { x // x ∈ closure (id '' K) } → (K.topologicalClosure : Set ℂ) :=
      fun x =>
      ⟨x, by
        convert! x.prop
        simp only [id, Set.image_id']
        rfl ⟩
    convert!
      DenseRange.comp (Function.Surjective.denseRange _) (IsDenseEmbedding.id.subtype (· ∈ K)).dense
        (by fun_prop : Continuous j)
    rintro ⟨y, hy⟩
    use
      ⟨y, by
        convert! hy
        simp only [id, Set.image_id']
        rfl ⟩

Goal state after: