Rails/RedundantAllowNil Cop gives false positives under some specific scenarios.

[gPrado]

The Rails/RedundantAllowNil gives false positives when allow_nil/allow_blank configuration are used in nested validations.

In the example below the requirements are:

• email field must accept nil values;
• email field must not accept empty string values. The failure message must be "can't be blank" and that must be the only failure message;
• email field must be validated against a regular expression. The failure message must be "is invalid" and that must be the only failure message;

The allow_blank is therefore not redundant in the nested scope, because any other configuration would break at least one of the requirements above.

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Steps to reproduce the problem

Given the following code:

class User
  include ActiveModel::Validations

  attr_accessor :email

  validates :email,
    format: {
      with: /some email address regexp/,
      allow_blank: true
    },
    presence: {
      allow_nil: true
    }
end

or

class User
  include ActiveModel::Validations

  attr_accessor :email

  validates :email,
    format: {
      with: /some email address regexp/,
      allow_blank: true
    },
    presence: true,
    allow_nil: true
end

Expected behavior

Rubocop should not raise errors.

Actual behavior

Rubocop fails with the following message: "Rails/RedundantAllowNil: allow_nil is redundant when allow_blank has the same value."

RuboCop version

0.93.1 (using Parser 2.7.2.0, rubocop-ast 0.8.0, running on ruby 2.5.8 x86_64-linux)