def1.lean:5:16: error: "eliminator" elaborator type mismatch, term
  rfl
has type
  ?m_2 = ?m_2
but is expected to have type
  f b = f c
Additional information:
def1.lean:5:16: context: the inferred motive for the eliminator-like application is
  λ (_x : A), f _x = f c