-- Mathlib/Topology/Instances/Complex.lean
theorem Complex.uniformContinuous_ringHom_eq_id_or_conj (K : Subfield ℂ) {ψ : K →+* ℂ}
    (hc : UniformContinuous ψ) : ψ.toFun = K.subtype ∨ ψ.toFun = conj ∘ K.subtype

Proof state:
case refine_2.inr
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
di : IsDenseInducing ι := IsUniformInducing.isDenseInducing ui ?refine_1
extψ : ↥K.topologicalClosure →+* ℂ := IsDenseInducing.extendRingHom ui ⋯ hc
hψ : Continuous (⋯.extend ⇑ψ)
h : K.topologicalClosure = ⊤
ψ₁ : ℂ →+* ℂ := extψ.comp ((RingEquiv.subfieldCongr h).symm.toRingHom.comp Subfield.topEquiv.symm.toRingHom)
hψ₁ : Continuous ⇑ψ₁
⊢ (↑↑ψ).toFun = ⇑K.subtype ∨ (↑↑ψ).toFun = ⇑(starRingEnd ℂ) ∘ ⇑K.subtype

Tactic:
rcases ringHom_eq_id_or_conj_of_continuous hψ₁ with h | h

Result:
case refine_2.inr.inl
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
di : IsDenseInducing ι := IsUniformInducing.isDenseInducing ui ?refine_1
extψ : ↥K.topologicalClosure →+* ℂ := IsDenseInducing.extendRingHom ui ⋯ hc
hψ : Continuous (⋯.extend ⇑ψ)
h✝ : K.topologicalClosure = ⊤
ψ₁ : ℂ →+* ℂ := extψ.comp ((RingEquiv.subfieldCongr h✝).symm.toRingHom.comp Subfield.topEquiv.symm.toRingHom)
hψ₁ : Continuous ⇑ψ₁
h : ψ₁ = RingHom.id ℂ
⊢ (↑↑ψ).toFun = ⇑K.subtype ∨ (↑↑ψ).toFun = ⇑(starRingEnd ℂ) ∘ ⇑K.subtype

case refine_2.inr.inr
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
di : IsDenseInducing ι := IsUniformInducing.isDenseInducing ui ?refine_1
extψ : ↥K.topologicalClosure →+* ℂ := IsDenseInducing.extendRingHom ui ⋯ hc
hψ : Continuous (⋯.extend ⇑ψ)
h✝ : K.topologicalClosure = ⊤
ψ₁ : ℂ →+* ℂ := extψ.comp ((RingEquiv.subfieldCongr h✝).symm.toRingHom.comp Subfield.topEquiv.symm.toRingHom)
hψ₁ : Continuous ⇑ψ₁
h : ψ₁ = starRingEnd ℂ
⊢ (↑↑ψ).toFun = ⇑K.subtype ∨ (↑↑ψ).toFun = ⇑(starRingEnd ℂ) ∘ ⇑K.subtype

Proof state:
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
j : { x // x ∈ closure (id '' {x | ↑K x}) } → ↑↑K.topologicalClosure := fun x => ⟨↑x, ⋯⟩
y : ℂ
hy : y ∈ ↑K.topologicalClosure
⊢ y ∈ closure (id '' {x | ↑K x})

Tactic:
convert hy

Result:
case h.e'_4
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
j : { x // x ∈ closure (id '' {x | ↑K x}) } → ↑↑K.topologicalClosure := fun x => ⟨↑x, ⋯⟩
y : ℂ
hy : y ∈ ↑K.topologicalClosure
⊢ closure (id '' {x | ↑K x}) = ↑K.topologicalClosure

Proof state:
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
⊢ (↑↑ψ).toFun = ⇑K.subtype ∨ (↑↑ψ).toFun = ⇑(starRingEnd ℂ) ∘ ⇑K.subtype

Tactic:
set ι : K → K.topologicalClosure := ⇑(Subfield.inclusion K.le_topologicalClosure)

Result:
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
⊢ (↑↑ψ).toFun = ⇑K.subtype ∨ (↑↑ψ).toFun = ⇑(starRingEnd ℂ) ∘ ⇑K.subtype

Proof state:
case h.e'_2
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
di : IsDenseInducing ι := IsUniformInducing.isDenseInducing ui ?refine_1
extψ : ↥K.topologicalClosure →+* ℂ := IsDenseInducing.extendRingHom ui ⋯ hc
hψ : Continuous (⋯.extend ⇑ψ)
h✝ : K.topologicalClosure = ⊤
ψ₁ : ℂ →+* ℂ := extψ.comp ((RingEquiv.subfieldCongr h✝).symm.toRingHom.comp Subfield.topEquiv.symm.toRingHom)
hψ₁ : Continuous ⇑ψ₁
h : ψ₁ = starRingEnd ℂ
z : ↥K
⊢ (↑↑ψ).toFun z = ψ₁ ↑z

Tactic:
exact (IsDenseInducing.extend_eq di hc.continuous z).symm

Result:
no goals

Proof state:
case refine_1
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
j : { x // x ∈ closure (id '' {x | ↑K x}) } → ↑↑K.topologicalClosure := fun x => ⟨↑x, ⋯⟩
⊢ DenseRange ι

Tactic:
convert DenseRange.comp (Function.Surjective.denseRange _)
  (IsDenseEmbedding.id.subtype (K : Set ℂ)).dense (by continuity : Continuous j)

Result:
case refine_1
K : Subfield ℂ
ψ : ↥K →+* ℂ
hc : UniformContinuous ⇑ψ
this✝ : TopologicalDivisionRing ℂ := TopologicalDivisionRing.mk
this : IsTopologicalRing ↥K.topologicalClosure := Subring.instIsTopologicalRing K.topologicalClosure.toSubring
ι : ↥K → ↥K.topologicalClosure := ⇑(Subfield.inclusion ⋯)
ui : IsUniformInducing ι
j : { x // x ∈ closure (id '' {x | ↑K x}) } → ↑↑K.topologicalClosure := fun x => ⟨↑x, ⋯⟩
⊢ Function.Surjective j