[GHC] #14916: Missing checks when deriving special classes

13 Mar 2018

      #14916: Missing checks when deriving special classes
-------------------------------------+-------------------------------------
           Reporter:  kosmikus       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 For the following program
 {{{
 {-# LANGUAGE DeriveAnyClass #-}
 module T where

 import Data.Coerce
 import Data.Typeable

 data A = MkA deriving ((~) A)
 data B = MkB deriving (Coercible B)
 }}}
 the deriving clause for `A` is accepted without complaints, and the
 deriving clause for `B` fails with the following error:
 {{{
 T.hs:8:24: error:
     Top-level bindings for unlifted types aren't allowed:
   |
 8 | data B = MkB deriving (Coercible B)
   |                        ^^^^^^^^^^^
 }}}

 Corresponding standalone deriving instances trigger errors
 saying "Manual instances of this class are not permitted".
 Probably similar error messages should be triggered here.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14916
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