implicitArgumentError.lean:18:6-18:9: error: don't know how to synthesize implicit argument `n`
  @foo ?_
context:
⊢ Nat
implicitArgumentError.lean:16:0-16:11: error: ❌️ Docstring on `#guard_msgs` does not match generated message:

- error: don't know how to synthesize implicit argument 'n'
+ error: don't know how to synthesize implicit argument `n`
    @foo ?_
  context:
  ⊢ Nat