Weird edge case allowing polymorphic exceptions in ReasonML/OCaml

PolymorphicExceptions.re

/* Uncomment below line to see error       */
/* exception Absurd('a);                   */
/* ^^ Error: Unbound type parameter 'a     */

/*
Normally, exceptions are monomorphic. They usually cannot take in type params...
*/

/*
...But what if we skip the `exception` syntax sugar and just use the `+=` open 
variant extension syntax in tandem with GADT syntax? Aha! We've done the impossible!
*/
type exn +=
  | Absurd('a) : exn;

let absurdString = Absurd("lol");
let absurdInt = Absurd(0);
let absurdPolyVar = Absurd([ `LolWut ]);

raise(absurdString);
/* raise(absurdInt); */
/* raise(absurdPolyVar); */