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, 6 insertions, 1 deletions

diff --git a/compiler/rename/RnSource.hs b/compiler/rename/RnSource.hs
index 7a205ba3b9..6027110f36 100644
--- a/compiler/rename/RnSource.hs
+++ b/compiler/rename/RnSource.hs
@@ -738,7 +738,12 @@ rnFamInstEqn doc mb_cls rhs_kvars
        ; ((bndrs', pats', payload'), fvs)
               <- bindLocalNamesFV all_imp_var_names $
                  bindLHsTyVarBndrs doc (Just $ inHsDocContext doc)
-                                   mb_cls bndrs $ \bndrs' ->
+                                   Nothing bndrs $ \bndrs' ->
+                 -- Note: If we pass mb_cls instead of Nothing here,
+                 --  bindLHsTyVarBndrs will use class variables for any names
+                 --  the user meant to bring in scope here. This is an explicit
+                 --  forall, so we want fresh names, not class variables.
+                 --  Thus: always pass Nothing
                  do { (pats', pat_fvs) <- rnLHsTypes (FamPatCtx tycon) pats
                     ; (payload', rhs_fvs) <- rn_payload doc payload