test.rb

class TestCase1
  if false
    def foo
      'Hello World'
    end
  end
end

puts TestCase1.new.foo # test.rb:9:in `<main>': undefined method `foo' for #<TestCase1:0x007ffff35bfe78> (NoMethodError)

class TestCase2
  if true
    def foo
      'Hello World'
    end
  end
end

puts TestCase2.new.fo() # test.rb:19:in `<main>': undefined method `fo' for #<TestCase2:0x007fffbcd27f80> (NoMethodError)
# Did you mean?  foo