Make error when deriving an instance for a typeclass less misleading

Before, when you attempted to derive an instance for a typeclass,
e.g.,

```
class C1 (a :: Constraint) where
class C2 where

deriving instance C1 C2
```

GHC would complain that `C2`'s data constructors aren't in scope. But
that
makes no sense, since typeclasses don't have constructors! By refining
the
checks that GHC performs when deriving, we can make the error message a
little more sensible.

This also cleans up a related `DeriveAnyClass` infelicity. Before, you
wouldn't have been able to compile code like this:

```
import System.IO (Handle)
class C a
deriving instance C Handle
```

Since GHC was requiring that all data constructors of `Handle` be in
scope. But `DeriveAnyClass` doesn't even generate code that mentions
any data constructors, so this requirement is silly!

Fixes #11509.

Test Plan: make test TEST=T11509

Reviewers: simonpj, austin, bgamari

Reviewed By: simonpj, bgamari

Subscribers: thomie, simonpj

Differential Revision: https://phabricator.haskell.org/D2558

GHC Trac Issues: #11509

1 files changed, 15 insertions, 1 deletions

diff --git a/docs/users_guide/8.2.1-notes.rst b/docs/users_guide/8.2.1-notes.rst
index 2147dbca1d..c176a0867a 100644
--- a/docs/users_guide/8.2.1-notes.rst
+++ b/docs/users_guide/8.2.1-notes.rst
@@ -42,6 +42,20 @@ Compiler
   class instance using the :ghc-flag:`-XDerivingStrategies` language extension
   (see :ref:`deriving-strategies`).
 
+- GHC now allows standalone deriving using :ghc-flag:`-XDeriveAnyClass` on
+  any data type, even if its data constructors are not in scope. This is
+  consistent with the fact that this code (in the presence of
+  :ghc-flag:`-XDeriveAnyClass`): ::
+
+      deriving instance C T
+
+  is exactly equivalent to: ::
+
+      instance C T
+
+  and the latter code has no restrictions about whether the data constructors
+  of ``T`` are in scope.
+
 GHCi
 ~~~~
 
@@ -159,7 +173,7 @@ filepath
 ghc
 ~~~
 
--  
+-
 ghc-boot
 ~~~~~~~~