The following works fine:

type foo
type bar
type _ t =
| Foo : string -> foo t
| Bar : string -> bar t

let to_string : type a . a t -> string = function
  | Foo x -> x
  | Bar x -> x


However, if you try to avoid the redundant code of the two branches, you
get a compile error:

let to_string : type a . a t -> string = function
  | Foo x
  | Bar x -> x

Error: This pattern matches values of type foo t
       but a pattern was expected which matches values of type a t
       Type foo is not compatible with type a

Is there a real reason for this?