Unexpected '_a problem

["Chris King" <colanderman@gmail.com>]

Given this code:

# let create_foo () = object method foo (_: 'a) = () end;;
val create_foo : unit -> < foo : 'a -> unit > = <fun>
# let a = create_foo ();;
val a : < bar : '_a -> unit > = <obj>

the compiler determines a is monomorphic, since 'a is in a
contravariant position.  But why, when 'a is placed in a covariant
position:

# let create_bar () = object method bar (_: 'a) = () end;;
val create_bar : unit -> < bar : ('a -> unit) -> unit > = <fun>
# let b = create_bar ();;
val b : < bar : ('_a -> unit) -> unit > = <obj>

does the compiler restrict b to be monomorphic?  If I wrap everything
up in a module and abstract the type returned by create_bar (),
create_bar () works as expected and returns a polymorphic value.

Thanks,
Chris King