/-- Auxiliary lemma for `Lp.induction_stronglyMeasurable`. -/
@[elab_as_elim]
theorem Lp.induction_stronglyMeasurable_aux (hm : m ≤ m0) (hp_ne_top : p ≠ ∞) (P : Lp F p μ → Prop)
    (h_ind : ∀ (c : F) {s : Set α} (hs : MeasurableSet[m] s) (hμs : μ s < ∞),
      P (Lp.simpleFunc.indicatorConst p (hm s hs) hμs.ne c))
    (h_add : ∀ ⦃f g⦄, ∀ hf : Memℒp f p μ, ∀ hg : Memℒp g p μ, ∀ _ : AEStronglyMeasurable' m f μ,
      ∀ _ : AEStronglyMeasurable' m g μ, Disjoint (Function.support f) (Function.support g) →
        P (hf.toLp f) → P (hg.toLp g) → P (hf.toLp f + hg.toLp g))
    (h_closed : IsClosed {f : lpMeas F ℝ m p μ | P f}) :
    ∀ f : Lp F p μ, AEStronglyMeasurable' m f μ → P f := by
  intro f hf
  let f' := (⟨f, hf⟩ : lpMeas F ℝ m p μ)
  let g := lpMeasToLpTrimLie F ℝ p μ hm f'
  have hfg : f' = (lpMeasToLpTrimLie F ℝ p μ hm).symm g := by
    simp only [LinearIsometryEquiv.symm_apply_apply]
  change P ↑f'
  rw [hfg]
  refine'
    @Lp.induction α F m _ p (μ.trim hm) _ hp_ne_top
      (fun g => P ((lpMeasToLpTrimLie F ℝ p μ hm).symm g)) _ _ _ g
  · intro b t ht hμt
    -- Porting note: needed to pass `m` to `Lp.simpleFunc.coe_indicatorConst` to avoid
    -- synthesized type class instance is not definitionally equal to expression inferred by typing
    -- rules, synthesized m0 inferred m
    rw [@Lp.simpleFunc.coe_indicatorConst _ _ m, lpMeasToLpTrimLie_symm_indicator ht hμt.ne b]
    have hμt' : μ t < ∞ := (le_trim hm).trans_lt hμt
    specialize h_ind b ht hμt'
    rwa [Lp.simpleFunc.coe_indicatorConst] at h_ind
  · intro f g hf hg h_disj hfP hgP
    rw [LinearIsometryEquiv.map_add]
    push_cast
    have h_eq :
      ∀ (f : α → F) (hf : Memℒp f p (μ.trim hm)),
        ((lpMeasToLpTrimLie F ℝ p μ hm).symm (Memℒp.toLp f hf) : Lp F p μ) =
          (memℒp_of_memℒp_trim hm hf).toLp f :=
      lpMeasToLpTrimLie_symm_toLp hm
    rw [h_eq f hf] at hfP ⊢
    rw [h_eq g hg] at hgP ⊢
    exact
      h_add (memℒp_of_memℒp_trim hm hf) (memℒp_of_memℒp_trim hm hg)
        (aeStronglyMeasurable'_of_aeStronglyMeasurable'_trim hm hf.aestronglyMeasurable)
        (aeStronglyMeasurable'_of_aeStronglyMeasurable'_trim hm hg.aestronglyMeasurable)
        h_disj hfP hgP
  · 

Proof state:
case refine'_3
α : Type u_1
E' : Type u_2
F : Type u_3
F' : Type u_4
𝕜 : Type u_5
p : ℝ≥0∞
inst✝¹² : IsROrC 𝕜
inst✝¹¹ : NormedAddCommGroup E'
inst✝¹⁰ : InnerProductSpace 𝕜 E'
inst✝⁹ : CompleteSpace E'
inst✝⁸ : NormedSpace ℝ E'
inst✝⁷ : NormedAddCommGroup F
inst✝⁶ : NormedSpace 𝕜 F
inst✝⁵ : NormedAddCommGroup F'
inst✝⁴ : NormedSpace 𝕜 F'
inst✝³ : NormedSpace ℝ F'
inst✝² : CompleteSpace F'
m m0 : MeasurableSpace α
μ : Measure α
inst✝¹ : Fact (1 ≤ p)
inst✝ : NormedSpace ℝ F
hm : m ≤ m0
hp_ne_top : p ≠ ⊤
P : ↥(Lp F p) → Prop
h_ind :
  ∀ (c : F) {s : Set α} (hs : MeasurableSet s) (hμs : ↑↑μ s < ⊤),
    P ↑(simpleFunc.indicatorConst p (_ : MeasurableSet s) (_ : ↑↑μ s ≠ ⊤) c)
h_add :
  ∀ ⦃f g : α → F⦄ (hf : Memℒp f p) (hg : Memℒp g p),
    AEStronglyMeasurable' m f μ →
      AEStronglyMeasurable' m g μ →
        Disjoint (Function.support f) (Function.support g) →
          P (Memℒp.toLp f hf) → P (Memℒp.toLp g hg) → P (Memℒp.toLp f hf + Memℒp.toLp g hg)
h_closed : IsClosed {f | P ↑f}
f : ↥(Lp F p)
hf : AEStronglyMeasurable' m (↑↑f) μ
f' : ↥(lpMeas F ℝ m p μ) := { val := f, property := hf }
g : ↥(Lp F p) := (lpMeasToLpTrimLie F ℝ p μ hm) f'
hfg : f' = (LinearIsometryEquiv.symm (lpMeasToLpTrimLie F ℝ p μ hm)) g
⊢ IsClosed {f | (fun g => P ↑((LinearIsometryEquiv.symm (lpMeasToLpTrimLie F ℝ p μ hm)) g)) f}

Next tactic:
change IsClosed ((lpMeasToLpTrimLie F ℝ p μ hm).symm ⁻¹' {g : lpMeas F ℝ m p μ | P ↑g})