/--
Given `R`-module X and `S`-module Y and a map `X ⟶ (restrictScalars f).obj Y`, i.e `R`-linear map
`X ⟶ Y`, there is a map `(extend_scalars f).obj X ⟶ Y`, i.e `S`-linear map `S ⨂ X → Y` by
`s ⊗ x ↦ s • g x`.
-/
@[simps apply]
def HomEquiv.fromExtendScalars {X Y} (g : X ⟶ (restrictScalars f).obj Y) :
    (extendScalars f).obj X ⟶ Y := by
  letI m1 : Module R S := Module.compHom S f; letI m2 : Module R Y := Module.compHom Y f
  refine {toFun := fun z => TensorProduct.lift ?_ z, map_add' := ?_, map_smul' := ?_}
  · refine
    {toFun := fun s => HomEquiv.evalAt f s g, map_add' := fun (s₁ s₂ : S) => ?_,
      map_smul' := fun (r : R) (s : S) => ?_}
    · ext
      dsimp
      rw [← add_smul]
    · ext x
      apply mul_smul (f r) s (g x)
  · intros z₁ z₂
    

Proof state:
case refine_2
R✝ : Type u₁
S✝ : Type u₂
inst✝³ : CommRing R✝
inst✝² : CommRing S✝
f✝ : R✝ →+* S✝
R : Type u₁
S : Type u₂
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
X : ModuleCat R
Y : ModuleCat S
g : X ⟶ (restrictScalars f).obj Y
m1 : Module R S := Module.compHom S f
m2 : Module R ↑Y := Module.compHom (↑Y) f
z₁ z₂ : ↑((extendScalars f).obj X)
⊢ (fun z =>
        (lift
            {
              toAddHom :=
                { toFun := fun s => evalAt f s g,
                  map_add' :=
                    (_ :
                      ∀ (s₁ s₂ : S),
                        (fun s => evalAt f s g) (s₁ + s₂) = (fun s => evalAt f s g) s₁ + (fun s => evalAt f s g) s₂) },
              map_smul' :=
                (_ :
                  ∀ (r : R) (s : S),
                    AddHom.toFun
                        { toFun := fun s => evalAt f s g,
                          map_add' :=
                            (_ :
                              ∀ (s₁ s₂ : S),
                                (fun s => evalAt f s g) (s₁ + s₂) =
                                  (fun s => evalAt f s g) s₁ + (fun s => evalAt f s g) s₂) }
                        (r • s) =
                      (RingHom.id R) r •
                        AddHom.toFun
                          { toFun := fun s => evalAt f s g,
                            map_add' :=
                              (_ :
                                ∀ (s₁ s₂ : S),
                                  (fun s => evalAt f s g) (s₁ + s₂) =
                                    (fun s => evalAt f s g) s₁ + (fun s => evalAt f s g) s₂) }
                          s) })
          z)
      (z₁ + z₂) =
    (fun z =>
          (lift
              {
                toAddHom :=
                  { toFun := fun s => evalAt f s g,
                    map_add' :=
                      (_ :
                        ∀ (s₁ s₂ : S),
                          (fun s => evalAt f s g) (s₁ + s₂) =
                            (fun s => evalAt f s g) s₁ + (fun s => evalAt f s g) s₂) },
                map_smul' :=
                  (_ :
                    ∀ (r : R) (s : S),
                      AddHom.toFun
                          { toFun := fun s => evalAt f s g,
                            map_add' :=
                              (_ :
                                ∀ (s₁ s₂ : S),
                                  (fun s => evalAt f s g) (s₁ + s₂) =
                                    (fun s => evalAt f s g) s₁ + (fun s => evalAt f s g) s₂) }
                          (r • s) =
                        (RingHom.id R) r •
                          AddHom.toFun
                            { toFun := fun s => evalAt f s g,
                              map_add' :=
                                (_ :
                                  ∀ (s₁ s₂ : S),
                                    (fun s => evalAt f s g) (s₁ + s₂) =
                                      (fun s => evalAt f s g) s₁ + (fun s => evalAt f s g) s₂) }
                            s) })
            z)
        z₁ +
      (fun z =>
          (lift
              {
                toAddHom :=
                  { toFun := fun s => evalAt f s g,
                    map_add' :=
                      (_ :
                        ∀ (s₁ s₂ : S),
                          (fun s => evalAt f s g) (s₁ + s₂) =
                            (fun s => evalAt f s g) s₁ + (fun s => evalAt f s g) s₂) },
                map_smul' :=
                  (_ :
                    ∀ (r : R) (s : S),
                      AddHom.toFun
                          { toFun := fun s => evalAt f s g,
                            map_add' :=
                              (_ :
                                ∀ (s₁ s₂ : S),
                                  (fun s => evalAt f s g) (s₁ + s₂) =
                                    (fun s => evalAt f s g) s₁ + (fun s => evalAt f s g) s₂) }
                          (r • s) =
                        (RingHom.id R) r •
                          AddHom.toFun
                            { toFun := fun s => evalAt f s g,
                              map_add' :=
                                (_ :
                                  ∀ (s₁ s₂ : S),
                                    (fun s => evalAt f s g) (s₁ + s₂) =
                                      (fun s => evalAt f s g) s₁ + (fun s => evalAt f s g) s₂) }
                            s) })
            z)
        z₂

Next tactic:
change lift _ (z₁ + z₂) = lift _ z₁ + lift _ z₂