type (_, _, _) desc =
    | L : ('a, _, 'a) desc
    | R : (_, 'b, 'b) desc

type (_, _, _) t =
    Tee : ('a, 'b, 'e) desc * ('e -> 't) -> ('a, 'b, 't) t

let id x = x

(* this works as expected *)
let test_1 : type a b c . (a, b, c) desc -> (a, b, c) t =
    function
    | L -> Tee (L, id)
    | R -> Tee (R, id)

(* this will cause type error *)
let test_2 : 'a 'b 'c . ('a, 'b, 'c) desc -> ('a, 'b, 'c) t =
    function
    | L -> Tee (L, id)
    | R -> Tee (R, id)