[GHC] #11509: Incorrect error message in StandaloneDeriving: "The data constructors of <typeclass> are not all in scope"

[GHC]

#11509: Incorrect error message in StandaloneDeriving: "The data constructors of
<typeclass> are not all in scope"
-------------------------------------+-------------------------------------
        Reporter:  edsko             |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2558
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"d5a4e49d657682eeb6e86ae464d281974ce2f5e2/ghc"
 d5a4e49d/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="d5a4e49d657682eeb6e86ae464d281974ce2f5e2"
 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
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11509#comment:13>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler