Struct pattern in function head does not handle matching variables

[erszcz]

  # FIXME: It seems the second attempt/1 clause cannot be reached.
  @spec attempt(%Waiter{}) :: {:ok, %Waiter{}} | {:error, %Waiter{}}
  defp attempt(%Waiter{attempt_num: num, num_attempts: num} = waiter) do

This one seems to be caused by a known limitation in the type checker, i.e. by it not being able to understand variable binds in the struct pattern and the fact that both fields must match. In other words, the problem is on the type checker side, not in the checked code, so it's a false positive.

Originally reported by @baldwindavid in #84