Proposal: Add method annotations: primitive:nil? and primitive:not_nil?

[tk0miya]

To represent methods like Object#blank? and Object#present? in ActiveSupport, this adds method annotations primitive:nil? and primitive:not_nil?.

They will be considered as same as nil? and not nil? internally.

refs: #472

Please let me know your opinion about implementation, the naming of annotations, and so on.

[ParadoxV5]

Someone in Ruby Discord asked if we can write “truthy”, i.e. exact opposite of false | nil. Would that in the RBS core be more preferred?

[tk0miya]

The goal of this PR is to provide type narrowing to Steep. So this is not related to the "truthy" type that you mentioned.

For example, the following code is available in Ruby on Rails:

name = params[:name]  #=> String?
if name.present?
  name  #=> String
end

But current steep does not support this type narrowing.

This PR adds a new feature to define type narrowing methods via primitive:nil? and `primitive:not_nil? annotation.

[derikson]

If we defined #present? as:

class Object
  %a{pure} def present?: () -> bool
end

class NilClass
  %a{pure} def present?: () -> ::FalseClass
end

In the example below, shouldn't Steep have enough information to narrow name from String? to String inside the conditional?

name = params[:name]  #=> String?
if name.present?
  name  #=> String
end

This doesn't appear to work currently, but it seems like it should.

[mtsmfm]

Unlike nil?, blank? doesn't guarantee the receiver is nil (e.g. it returns true for empty string)