F* exits when defining type class instance for let recs

[andricicezar]

F* exits when trying to type check this code:

open FStar.Tactics
open FStar.Tactics.Typeclasses

class mytc (#a:Type0) (s:a) = {  e : unit; }

instance mytc_let_rec () : mytc (let rec f x = x in ()) = { e = () }