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, 2 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_fail/all.T b/testsuite/tests/deriving/should_fail/all.T
index ce0cc0f155..5fec71eff5 100644
--- a/testsuite/tests/deriving/should_fail/all.T
+++ b/testsuite/tests/deriving/should_fail/all.T
@@ -64,5 +64,7 @@ test('T10598_fail3', normal, compile_fail, [''])
 test('T10598_fail4', normal, compile_fail, [''])
 test('T10598_fail5', normal, compile_fail, [''])
 test('T10598_fail6', normal, compile_fail, [''])
+test('T11509_1', [when(doing_ghci(), extra_hc_opts('-fobject-code'))],
+     compile_fail, [''])
 test('T12163', normal, compile_fail, [''])
 test('T12512', omit_ways(['ghci']), compile_fail, [''])