simp [List.filterMap] at h
case cons
α✝¹ : Type u_1
α✝ : Type u_2
f : α✝¹ → Option α✝
x y : α✝¹
ys : List α✝¹
ih : List.filterMap f ys = [] → x ∈ ys → f x = none
h : List.filterMap f (y :: ys) = []
hx : x ∈ y :: ys
⊢ f x = none
case cons
α✝¹ : Type u_1
α✝ : Type u_2
f : α✝¹ → Option α✝
x y : α✝¹
ys : List α✝¹
ih : List.filterMap f ys = [] → x ∈ ys → f x = none
h :
  (match f y with
    | none => List.filterMap f ys
    | some b => b :: List.filterMap f ys) =
    []
hx : x ∈ y :: ys
⊢ f x = none