Naively forbidding inheritance from classes defining tests

minitest_parent.rb

require 'minitest/autorun'

module ForbidSubclassingIfTestMethodsDefined
  DuplicateTestsError = Class.new(StandardError)
  def inherited(klass)
    unless runnable_methods.empty?
      raise DuplicateTestsError, "Do not inherit from #{self}, as the tests it defines would be duplicated in #{klass}"
    end

    super
  end
end

Minitest::Test.extend(ForbidSubclassingIfTestMethodsDefined)

class Grandparent < Minitest::Test
  # no tests
end

class Parent < Grandparent # okay
  def test_example
  end
end

class Child < Parent # boom
end

# minitest_parent.rb:7:in `inherited':
# Do not inherit from Parent, as the tests it defines would be duplicated in Child
# (ForbidSubclassingIfTestMethodsDefined::DuplicateTestsError)
#   from minitest_parent.rb:25:in `<main>'