Oddness with recursive polymorphic variants

[Jeremy Yallop <j.d.yallop@sms.ed.ac.uk>]

I have two polymorphic variant types, as follows:

   type f = [`A | `B of f]
   type g = [f | `C]

Next, I have a function from f to g:

   let s1 : f -> g = function
     | `A -> `A
     | `B b -> b

Sadly, the compiler rejects this:

     Characters 57-58:
       | `B b -> b;;
                 ^
   This expression has type f but is here used with type g
   The first variant type does not allow tag(s) `C

The error message seems odd.  Why should it matter that g has more tags 
than f, since every value of f is a value of g (by definition)?

Indeed, minor variants of the function are accepted.  Both of the 
following are ok:

   let s2 : f -> g = function
     | `A -> `A
     | `B (#f as b) -> b

   let s3 : f -> g = function
     | `A -> `A
     | `B ((`A|`B _) as b) -> b

Am I missing something, or is this a bug?

Thanks,

Jeremy.