From: SerP <serp@stork.ru>

> but if we use simple variant type instead of polymorphic variants it 
> works perfect
> 
> 
> type 'a get_mode = [ Read | Watch of (#getter as 'a)];;

(Better to avoid revised syntax in such cases, it is confusing here)

I was not aware of that, but this is logical since the problem comes
from expansion, and simple variants do not need to be expanded.
So I should correct my phrasing: explicit polymorphic type variables
inside constrained type parameters of type abbreviations are not
supported.

Jacques Garrigue