f : A → A
def4.lean:10:8: error: type mismatch at application
  f 0
term
  0
has type
  ℕ
but is expected to have type
  A
g : A → A
def4.lean:18:8: error: type mismatch at application
  g 0
term
  0
has type
  ℕ
but is expected to have type
  A
f : Π A, A → A
f ℕ 0 : ℕ
g 0 : ℕ