-- Mathlib/Algebra/Lie/Basic.lean
theorem toLinearEquiv_injective : Injective ((↑) : (L₁ ≃ₗ⁅R⁆ L₂) → L₁ ≃ₗ[R] L₂)

Proof state:
case mk.mk.mk.mk.mk.mk.mk.mk
R : Type u
L₁ : Type v
L₂ : Type w
inst✝⁴ : CommRing R
inst✝³ : LieRing L₁
inst✝² : LieRing L₂
inst✝¹ : LieAlgebra R L₁
inst✝ : LieAlgebra R L₂
f_inv : L₂ → L₁
f : L₁ → L₂
map_add'✝¹ : ∀ (x y : L₁), f (x + y) = f x + f y
map_smul'✝¹ :
  ∀ (m : R) (x : L₁),
    { toFun := f, map_add' := map_add'✝¹ }.toFun (m • x) =
      (RingHom.id R) m • { toFun := f, map_add' := map_add'✝¹ }.toFun x
map_lie'✝¹ :
  ∀ {x y : L₁},
    { toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹ }.toFun ⁅x, y⁆ =
      ⁅{ toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹ }.toFun x,
        { toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹ }.toFun y⁆
left_inv✝¹ :
  LeftInverse f_inv (↑{ toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹, map_lie' := map_lie'✝¹ }).toFun
right_inv✝¹ :
  RightInverse f_inv (↑{ toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹, map_lie' := map_lie'✝¹ }).toFun
g_inv : L₂ → L₁
g : L₁ → L₂
map_add'✝ : ∀ (x y : L₁), g (x + y) = g x + g y
map_smul'✝ :
  ∀ (m : R) (x : L₁),
    { toFun := g, map_add' := map_add'✝ }.toFun (m • x) =
      (RingHom.id R) m • { toFun := g, map_add' := map_add'✝ }.toFun x
map_lie'✝ :
  ∀ {x y : L₁},
    { toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝ }.toFun ⁅x, y⁆ =
      ⁅{ toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝ }.toFun x,
        { toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝ }.toFun y⁆
left_inv✝ :
  LeftInverse g_inv (↑{ toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝, map_lie' := map_lie'✝ }).toFun
right_inv✝ :
  RightInverse g_inv (↑{ toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝, map_lie' := map_lie'✝ }).toFun
h :
  { toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹, map_lie' := map_lie'✝¹, invFun := f_inv,
        left_inv := left_inv✝¹, right_inv := right_inv✝¹ }.toLinearEquiv =
    { toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝, map_lie' := map_lie'✝, invFun := g_inv,
        left_inv := left_inv✝, right_inv := right_inv✝ }.toLinearEquiv
⊢ { toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹, map_lie' := map_lie'✝¹, invFun := f_inv,
      left_inv := left_inv✝¹, right_inv := right_inv✝¹ } =
    { toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝, map_lie' := map_lie'✝, invFun := g_inv,
      left_inv := left_inv✝, right_inv := right_inv✝ }

Tactic:
simp only [toLinearEquiv_mk, LinearEquiv.mk.injEq, LinearMap.mk.injEq, AddHom.mk.injEq] at h

Result:
case mk.mk.mk.mk.mk.mk.mk.mk
R : Type u
L₁ : Type v
L₂ : Type w
inst✝⁴ : CommRing R
inst✝³ : LieRing L₁
inst✝² : LieRing L₂
inst✝¹ : LieAlgebra R L₁
inst✝ : LieAlgebra R L₂
f_inv : L₂ → L₁
f : L₁ → L₂
map_add'✝¹ : ∀ (x y : L₁), f (x + y) = f x + f y
map_smul'✝¹ :
  ∀ (m : R) (x : L₁),
    { toFun := f, map_add' := map_add'✝¹ }.toFun (m • x) =
      (RingHom.id R) m • { toFun := f, map_add' := map_add'✝¹ }.toFun x
map_lie'✝¹ :
  ∀ {x y : L₁},
    { toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹ }.toFun ⁅x, y⁆ =
      ⁅{ toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹ }.toFun x,
        { toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹ }.toFun y⁆
left_inv✝¹ :
  LeftInverse f_inv (↑{ toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹, map_lie' := map_lie'✝¹ }).toFun
right_inv✝¹ :
  RightInverse f_inv (↑{ toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹, map_lie' := map_lie'✝¹ }).toFun
g_inv : L₂ → L₁
g : L₁ → L₂
map_add'✝ : ∀ (x y : L₁), g (x + y) = g x + g y
map_smul'✝ :
  ∀ (m : R) (x : L₁),
    { toFun := g, map_add' := map_add'✝ }.toFun (m • x) =
      (RingHom.id R) m • { toFun := g, map_add' := map_add'✝ }.toFun x
map_lie'✝ :
  ∀ {x y : L₁},
    { toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝ }.toFun ⁅x, y⁆ =
      ⁅{ toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝ }.toFun x,
        { toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝ }.toFun y⁆
left_inv✝ :
  LeftInverse g_inv (↑{ toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝, map_lie' := map_lie'✝ }).toFun
right_inv✝ :
  RightInverse g_inv (↑{ toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝, map_lie' := map_lie'✝ }).toFun
h : f = g ∧ f_inv = g_inv
⊢ { toFun := f, map_add' := map_add'✝¹, map_smul' := map_smul'✝¹, map_lie' := map_lie'✝¹, invFun := f_inv,
      left_inv := left_inv✝¹, right_inv := right_inv✝¹ } =
    { toFun := g, map_add' := map_add'✝, map_smul' := map_smul'✝, map_lie' := map_lie'✝, invFun := g_inv,
      left_inv := left_inv✝, right_inv := right_inv✝ }