Note [Pattern signature binders and scoping]

Consider the pattern signatures like those on `t` and `g` in:

   f = let h = \(t :: (b, b) ->
               \(g :: forall a. a -> b) ->
               ...(t :: (Int,Int))...
       in woggle

* The `b` in t's pattern signature is implicitly bound and scopes over
  the signature and the body of the lambda.  It stands for a type (any type);
  indeed we subsequently discover that b=Int.
  (See Note [TyVarTv] in GHC.Tc.Utils.TcMType for more on this point.)
* The `b` in g's pattern signature is an /occurrence/ of the `b` bound by
  t's pattern signature.
* The `a` in `forall a` scopes only over the type `a -> b`, not over the body
  of the lambda.
* There is no forall-or-nothing rule for pattern signatures, which is why the
  type `forall a. a -> b` is permitted in `g`'s pattern signature, even though
  `b` is not explicitly bound. See Note [forall-or-nothing rule].

Similar scoping rules apply to term variable binders in RULES, like in the
following example:

   {-# RULES "h" forall (t :: (b, b)) (g :: forall a. a -> b). h t g = ... #