You should not be able to cast to a private record/variant

[tsnobip]

Today, this compiles without any warning or error (cf playground):

module Foo = {
  type t = private {foo: int}
}

module Bar = {
  type t = {...Foo.t}
}

let bar = {Bar.foo: 9}
let foo = (bar :> Foo.t)

which doesn't make sense because private records/variants are precisely meant to avoid instantiation.

This should IMO at least raise a warning.