> But this similar type has no problem:
>  type 'a field = Int of int
>  let a = Int 3
>  let b = Int (1 + 2)

This is the relaxed value restriction: `field` is covariant in it's
parameter which means that the type variable is only used in
covariant positions and so can be generalized even though the
expression is not a value.
> Both a and b receive the same generalized type. Why does the GADT
> style prevent this from occuring?
 OCaml currently just assumes that types defined with GADT syntax are
 invariant because checking of variance with GADTs is more difficult. If
 you annotate your type as covariant then it behaves the same:
  type +_ field = Int : int -> 'a field
  let a = Int 3
  let b = Int (1 + 2)

gives:

  type +_ field = Int : int -> 'a field
  val a : 'a field = Int 3
  val b : 'a field = Int 3

Regards,

Leo