Note [The polymorphism rule of join points]

Invariant 4 of Note [Invariants on join points] forbids a join point to be
polymorphic in its return type. That is, if its type is

  forall a1 ... ak. t1 -> ... -> tn -> r

where its join arity is k+n, none of the type parameters ai may occur free in r.

In some way, this falls out of the fact that given

  join
     j @a1 ... @ak x1 ... xn = e1
  in e2

then all calls to `j` are in tail-call positions of `e`, and expressions in
tail-call positions in `e` have the same type as `e`.
Therefore the type of `e1` -- the return type of the join point -- must be the
same as the type of e2.
Since the type variables aren't bound in `e2`, its type can't include them, and
thus neither can the type of `e1`.

This unfortunately prevents the `go` in the following code from being a
join-point:

  iter :: forall a. Int -> (a -> a) -> a -> a
  iter @a n f x = go @a n f x
    where
      go :: forall a. Int -> (a -> a) -> a -> a
      go @a 0 _ x = x
      go @a n f x = go @a (n-1) f (f x)

In this case, a static argument transformation would fix that (see
ticket #14620):

  iter :: forall a. Int -> (a -> a) -> a -> a
  iter @a n f x = go' @a n f x
    where
      go' :: Int -> (a -> a) -> a -> a
      go' 0 _ x = x
      go' n f x = go' (n-1) f (f x)

In general, loopification could be employed to do that (see #14068.)

Can we simply drop the requirement, and allow `go` to be a join-point? We
could, and it would work. But we could not longer apply the case-of-join-point
transformation universally. This transformation would do:

  case (join go @a n f x = case n of 0 -> x
                                     n -> go @a (n-1) f (f x)
        in go @Bool n neg True) of
    True -> e1; False -> e2

 ===>

  join go @a n f x = case n of 0 -> case x of True -> e1; False -> e2
                               n -> go @a (n-1) f (f x)
  in go @Bool n neg True

but that is ill-typed, as `x` is type `a`, not `Bool`.


This also justifies why we do not consider the `e` in `e |> co` to be in
tail position: A cast changes the type, but the type must be the same. But
operationally, casts are vacuous, so this is a bit unfortunate! See #14610 for
ideas how to fix this.