bad_error4.lean:5:0: error: type mismatch at application
  foo.mk (λ (a : unit) (b : delayed[?m_1]), delayed[?m_2])
term
  λ (a : unit) (b : delayed[?m_1]), delayed[?m_2]
has type
  Π (a : unit) (b : delayed[?m_1]), delayed[?m_3]
but is expected to have type
  unit → unit