open scoped Classical in
/-- The basis on `ι →₀ M` with basis vectors `λ ⟨i, x⟩, single i (b i x)`. -/
protected def basis {φ : ι → Type*} (b : ∀ i, Basis (φ i) R M) : Basis (Σi, φ i) R (ι →₀ M) :=
  Basis.ofRepr
    { toFun := fun g =>
        { toFun := fun ix => (b ix.1).repr (g ix.1) ix.2
          support := g.support.sigma fun i => ((b i).repr (g i)).support
          mem_support_toFun := fun ix => by
            simp only [Finset.mem_sigma, mem_support_iff, and_iff_right_iff_imp, Ne.def]
            intro b hg
            simp [hg] at b }
      invFun := fun g =>
        { toFun := fun i =>
            (b i).repr.symm (g.comapDomain _ (Set.injOn_of_injective sigma_mk_injective _))
          support := g.support.image Sigma.fst
          mem_support_toFun := fun i => by
            rw [Ne.def, ← (b i).repr.injective.eq_iff, (b i).repr.apply_symm_apply, FunLike.ext_iff]
            simp only [exists_prop, LinearEquiv.map_zero, comapDomain_apply, zero_apply,
              exists_and_right, mem_support_iff, exists_eq_right, Sigma.exists, Finset.mem_image,
              not_forall] }
      left_inv := fun g => by
        ext i
        rw [← (b i).repr.injective.eq_iff]
        ext x
        simp only [coe_mk, LinearEquiv.apply_symm_apply, comapDomain_apply]
      right_inv := fun g => by
        ext ⟨i, x⟩
        simp only [coe_mk, LinearEquiv.apply_symm_apply, comapDomain_apply]
      map_add' := fun g h => by
        ext ⟨i, x⟩
        simp only [coe_mk, add_apply, LinearEquiv.map_add]
      map_smul' := fun c h => by
        

Proof state:
R : Type u_1
M : Type u_2
ι : Type u_3
inst✝² : Semiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
φ : ι → Type u_4
b : (i : ι) → Basis (φ i) R M
c : R
h : ι →₀ M
⊢ AddHom.toFun
      {
        toFun := fun g =>
          { support := Finset.sigma g.support fun i => ((b i).repr (g i)).support,
            toFun := fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd,
            mem_support_toFun :=
              (_ :
                ∀ (ix : (i : ι) × φ i),
                  (ix ∈ Finset.sigma g.support fun i => ((b i).repr (g i)).support) ↔
                    (fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd) ix ≠ 0) },
        map_add' :=
          (_ :
            ∀ (g h : ι →₀ M),
              (fun g =>
                    { support := Finset.sigma g.support fun i => ((b i).repr (g i)).support,
                      toFun := fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd,
                      mem_support_toFun :=
                        (_ :
                          ∀ (ix : (i : ι) × φ i),
                            (ix ∈ Finset.sigma g.support fun i => ((b i).repr (g i)).support) ↔
                              (fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd) ix ≠ 0) })
                  (g + h) =
                (fun g =>
                      { support := Finset.sigma g.support fun i => ((b i).repr (g i)).support,
                        toFun := fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd,
                        mem_support_toFun :=
                          (_ :
                            ∀ (ix : (i : ι) × φ i),
                              (ix ∈ Finset.sigma g.support fun i => ((b i).repr (g i)).support) ↔
                                (fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd) ix ≠ 0) })
                    g +
                  (fun g =>
                      { support := Finset.sigma g.support fun i => ((b i).repr (g i)).support,
                        toFun := fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd,
                        mem_support_toFun :=
                          (_ :
                            ∀ (ix : (i : ι) × φ i),
                              (ix ∈ Finset.sigma g.support fun i => ((b i).repr (g i)).support) ↔
                                (fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd) ix ≠ 0) })
                    h) }
      (c • h) =
    (RingHom.id R) c •
      AddHom.toFun
        {
          toFun := fun g =>
            { support := Finset.sigma g.support fun i => ((b i).repr (g i)).support,
              toFun := fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd,
              mem_support_toFun :=
                (_ :
                  ∀ (ix : (i : ι) × φ i),
                    (ix ∈ Finset.sigma g.support fun i => ((b i).repr (g i)).support) ↔
                      (fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd) ix ≠ 0) },
          map_add' :=
            (_ :
              ∀ (g h : ι →₀ M),
                (fun g =>
                      { support := Finset.sigma g.support fun i => ((b i).repr (g i)).support,
                        toFun := fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd,
                        mem_support_toFun :=
                          (_ :
                            ∀ (ix : (i : ι) × φ i),
                              (ix ∈ Finset.sigma g.support fun i => ((b i).repr (g i)).support) ↔
                                (fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd) ix ≠ 0) })
                    (g + h) =
                  (fun g =>
                        { support := Finset.sigma g.support fun i => ((b i).repr (g i)).support,
                          toFun := fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd,
                          mem_support_toFun :=
                            (_ :
                              ∀ (ix : (i : ι) × φ i),
                                (ix ∈ Finset.sigma g.support fun i => ((b i).repr (g i)).support) ↔
                                  (fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd) ix ≠ 0) })
                      g +
                    (fun g =>
                        { support := Finset.sigma g.support fun i => ((b i).repr (g i)).support,
                          toFun := fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd,
                          mem_support_toFun :=
                            (_ :
                              ∀ (ix : (i : ι) × φ i),
                                (ix ∈ Finset.sigma g.support fun i => ((b i).repr (g i)).support) ↔
                                  (fun ix => ((b ix.fst).repr (g ix.fst)) ix.snd) ix ≠ 0) })
                      h) }
        h

Next tactic:
ext ⟨i, x⟩