Why can't I call a function over a subclass?

["Luca de Alfaro" <luca@dealfaro.org>]

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Consider the following sample code, that refuses to type check in Ocaml:

---start---

class r (x_init: int) = object
  method get_x : int = x_init
end

let f (r1: r) (r2: r) : bool = (r1#get_x = r2#get_x)

class r' (x_init: int) = object
  inherit r x_init
  method get_xx : int = 2 * x_init
end

let q (r1: r') (r2: r') = f r1 r2

---end---

r' is a subclass of r.  The compiler complains:

File "class1.ml", line 12, characters 28-30:
This expression has type r' but is here used with type r
The second object type has no method get_xx

Now, I know r does not have method get_xx, but why on Earth should this
matter?
I can see that there can be an error if I feed to f something that has FEWER
methods, but why should it be an error to feed to
f something with MORE methods??

The absurdity continues: if I instead declare f as:

let f (r1: <get_x: int; ..>) (r2: <get_x: int; ..>) : bool =
  (r1#get_x = r2#get_x)

then the code type-checks.  Why is this fine but f: r -> r -> bool causes a
type error?
The situation does not make sense to me.  Can someone shed some light on
what is going on?
(This with Ocaml 3.10.0 in case it matters)

Best,

Luca

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