Covariance issues with wrapping

[Drup]

While working with the ppx, I noticed something ... strange.

# let x = Html5.Xml.W.nil () ;;
val x : '_a list = []  
# let x : _ list = Html5.Xml.W.nil () ;;
val x : 'a list = []  

Apparently, the functor stack hides the covariance, since Xml_wrap.S.tlist is not marked covariant.
@vasilisp Do you think we could turn it covariant ? Are all the wrapper we have covariants ? Given the heavy use of subtyping, It would seems natural to enforce that.

[Drup]

As a limited fix, only for the native tyxml implementation, I did 85c62cd

[vasilisp]

Unfortunately, I much doubt that we can do it. To begin with, React.S.t is not marked covariant. Basically all our wrappers use mutation all over the place.

[Drup]

Mantis bug report: http://caml.inria.fr/mantis/view.php?id=7212

@vasilisp That is unfortunate. Remind me to check that mixing variances like that is actually sound (HTML-wise).