Add error when using type variables in an extend statement

[hjwylde]

We came across a case with rbs validate recently that we think should have an error:

class Foo[A]
  extend Bar[A]
end

module Bar[A]
end

We think that Bar[A] should have an error because extend is adding the methods to the singleton class, so it won't have access to instance type variables. This is similar to the other cases of singleton methods, class variables, etc.

[soutaro]

@hjwylde RBS 1.8.1 is out with an update for this. Thank you for this report!

[hjwylde]

Thanks @soutaro for the fast addition!