Hi Romain,

This is not a bug, but just a limitation of type inference on labels
-- it is independent from OO. If a type is unknown and is used for a
labelled application, the parameter will be supposed to be mandatory.
I guess inference goes too wild without that restriction -- because
with types where the optionality is unknown, it is also unknown when
an application is total.

A simpler example of that:

# let g f a = f ~a () ;;
val g : (a:'a -> unit -> 'b) -> 'a -> 'b = <fun>

The type of f is infered with a mandatory param labelled a.

# let f ?a b = b ;;
val f : ?a:'a -> 'b -> 'b = <fun>
# g f ;;
This expression has type ?a:'a -> 'b -> 'b but is here used with type
  a:'c -> unit -> 'd

This would work with an annotation in the declaration of g:

# let g (f : ?a:'a -> unit -> 'b) a = f ~a () ;;
val g : (?a:'a -> unit -> 'b) -> 'a -> 'b = <fun>

-- 
David