Does dyalizer need @spec when a function has @impl?

Questions & Help Questions
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1

Background

I have a module that is a GenStage. I am trying to make this module dyalizer compliant but I am not sure if I am going overboard with @spec.

Code

Imagine we have the following code:

@impl GenStage
def init(args) do
  {:producer_consumer, nil}
end

This code is the init callback of a GenStage (called after running start_link).

I wonder if I should let it be (because it is clear that it is already using the behaviour) or if I should add further information:

@spec init(any) :: {:producer_consumer, nil}
@impl GenStage
def init(args) do
  {:producer_consumer, nil}
end

Questions

  1. Which of the two version is correct?
  2. Which version would be better for dialyzer? (does it make a difference at all?)