gadt in 'guard' like pattern branch

output

File "test.ml", line 17, characters 13-32:
Error: This pattern matches values of type int prim expr
       but a pattern was expected which matches values of type a#0 expr
       Type int prim is not compatible with type a#0

test.ml

type _ prim =
  | TS : string -> int64 prim
  | Str : string -> string prim
  | Int : string -> int prim

type _ expr =
  | Val : 'a prim -> 'a prim expr
  | Expr : string * 'a expr * 'b expr -> 'c expr

let rec all : type x . x prim -> string = function
  | TS s -> s
  | Str s -> s
  | Int s -> s

let rec expr : type x . x expr -> string = function
  | Val x -> all x
  | Expr (s, (y : int prim expr), z) -> "guarded"
  | Expr (s, y, z) -> Printf.sprintf "%s -- %s -- %s" s (expr y) (expr z)