From: Keiko Nakata <keiko@kurims.kyoto-u.ac.jp>

> Why the following functor G is not typable?
> I thought that g could have a type something like int -> [< `A | `B ].
> 
> module G(X:sig type t = private [< `A ] val f : int -> t end) = struct
>   let g x = if x = 0 then `B else X.f x 
> end

Since g may return `B, and probably `A (the probably concerns your
intention), the expected type would be
  g : int -> [> `A | `B ]
Next, if you want f to have a private row type, then you should
disconnect its type from the type of g (otherwise the type of g must
be the same as f.) So I would expect something like:
   let g x = if x = 0 then `B else (X.f x : [< `A] :> [> `A])

Jacques