Fix #15828, from `More explicit foralls`

Summary:
Fix a bug in commit 12eeb9 which permits the following:

```
class C a where
  type T a b
instance C (Maybe a) where
  type forall a b. T (Maybe a) b = b
```

where instead, the user should write:

```
instance C (Maybe a) where
  type forall b. T (Maybe a) b = b
```

Update the users guide to discuss scoping of type variables in
explicit foralls in type family instances.

Test Plan: validate

Reviewers: bgamari, goldfire, monoidal

Reviewed By: goldfire

Subscribers: monoidal, rwbarton, carter

GHC Trac Issues: #15828

Differential Revision: https://phabricator.haskell.org/D5283

1 files changed, 3 insertions, 3 deletions

diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index a07adf377b..9b8df91916 100644
--- a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ -7577,9 +7577,9 @@ the left hand side can be explicitly bound. For example: ::
   
     data instance forall a (b :: Proxy a). F (Proxy b) = FProxy Bool
 
-When an explicit ``forall`` is present, all *type* variables mentioned must
-be bound by the ``forall``. Kind variables will be implicitly bound if
-necessary, for example: ::
+When an explicit ``forall`` is present, all *type* variables mentioned which
+are not already in scope must be bound by the ``forall``. Kind variables will
+be implicitly bound if necessary, for example: ::
   
     data instance forall (a :: k). F a = FOtherwise