[GHC] #15167: DerivClause list is not populated for (TyConI (DataD ...))

19 May 2018

      #15167: DerivClause list is not populated for (TyConI (DataD ...))
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           Reporter:  0xd34df00d     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Template       |           Version:  8.4.2
  Haskell                            |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!haskell
 % cat Test.hs
 {-# LANGUAGE LambdaCase #-}

 module Test where

 import Language.Haskell.TH

 test :: Name -> Q [Dec]
 test name = reify name >>= \case
     TyConI dec -> do
         runIO $ print dec
         pure []
     _ -> pure []

 % cat Run.hs
 {-# LANGUAGE TemplateHaskell #-}

 import Test

 data Foo = Foo deriving (Eq, Ord, Show)

 test ''Foo

 % ghc Run.hs
 [2 of 2] Compiling Main             ( Run.hs, Run.o )
 DataD [] Main.Foo [] Nothing [NormalC Main.Foo []] []
 }}}

 One might expect the `DataD` to mention `Eq, Ord, Show` in the
 `DerivClause` list, but it doesn't.

 This behavior manifests with every ghc version I tried: 8.0.2, 8.2.2,
 8.4.2. I also asked a question whether it's intended behaviour on
 #haskell, and I've been advised to open a bug report, so here it is.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15167
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