Thank you for the fast answer!

That means my second example will still be rejected in 4.08?

type 'a t =
  | A : [`A] t
  | B : [`A|`B] t

let f =
  fun (type a) ~(p :a t) () -> match p with
    | A -> (A: a t)
    | B -> (B: a t)

let g =
  fun (type a) ~(p :a t option) () -> match p with
    | Some B -> (B: a t)
    | Some A -> (A: a t)
    | None -> (A (* TYPE ERROR HERE *) : a t)

Error: This expression has type [ `A ] t
       but an expression was expected of type a t
       Type [ `A ] is not compatible with type a 


On Thu, 17 Jan 2019 11:12:02 +0100
Gabriel Scherer <gabriel.scherer@gmail.com> wrote:

> Matching on a constructor of a GADT may introduce a typing equation.
> In your example, matching on `A` introduces the equation `a = unit`.
> For this reason, the status of or-patterns (p1 | p2) containing GADT
> constructors is delicate: to type-check them we have to decide which
> equations from both sides are preserved in the result, computing a
> sort of intersection.
> 
> Released versions of OCaml avoid this difficulty by not supporting
> GADTs in or-patterns at all; you have to expand the pattern into two
> branches, as you did in your function `f` above.
> 
> In the current trunk, a change from Thomas Réfis and Leo White has
> been merged that allows GADTs in or-patterns, but discards the
> equations. Your specific example (the function `g`) is now accepted,
> and will be typeable in 4.08. Other examples, where you need to use
> an equation (provided by both sides of the or-patterns) in the body
> of the clause, will still be rejected.
> 
> 
> On Thu, Jan 17, 2019 at 10:46 AM Christopher Zimmermann <
> christopher@gmerlin.de> wrote:  
> 
> > Hi,
> >
> > why does the f type correctly while g fails to type?
> >
> > Christopher
> >
> > type 'a t =
> >   | A : unit t
> >
> > let f =
> >   fun (type a) ~(p :a t option) () -> match p with
> >     | Some A -> ()
> >     | None -> ()
> >
> > let g =
> >   fun (type a) ~(p :a t option) () -> match p with
> >     | Some A (* TYPING ERROR HERE *)
> >     | None -> ()
> >
> > Error: This pattern matches values of type unit t
> >        but a pattern was expected which matches values of type a t
> >        Type unit is not compatible with type a


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