Warning when deriving Foldable/Traversable using -Wall

[Simon Peyton Jones]

Yes, please add as a Trac ticket!  thank you

Simon

From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Alan & Kim Zimmerman
Sent: 08 October 2014 13:00
To: ghc-devs at haskell.org
Subject: Warning when deriving Foldable/Traversable using -Wall

I am not sure how to report bugs against the current development version of GHC.
Should this go into Trac?

The current HEAD gives a spurious unused declaration when deriving Typable/Traversable
Details

Compiling against current HEAD (0ed9a2779a2adf0347088134fdb9f60ae9f2735b)

Adding

  test('T9069w', extra_clean(['T9069.o', 'T9069.hi']), multimod_compile, ['T9069', '-Wall'])

to
  testsuite/tests/deriving/should_compile/all.T

results in

+[1 of 1] Compiling T9069            ( T9069.hs, T9069.o )
+
+T9069.hs:5:1: Warning:
+    The import of ‘Data.Foldable’ is redundant
+      except perhaps to import instances from ‘Data.Foldable’
+    To import instances alone, use: import Data.Foldable()
+
+T9069.hs:6:1: Warning:
+    The import of ‘Data.Traversable’ is redundant
+      except perhaps to import instances from ‘Data.Traversable’
+    To import instances alone, use: import Data.Traversable()
*** unexpected failure for T9069w(optasm)

The file being compiled is

--------------------------------------------
{-# LANGUAGE DeriveTraversable #-}

module T9069 where

import Data.Foldable
import Data.Traversable

data Trivial a = Trivial a
   deriving (Functor,Foldable,Traversable)
---------------------------------------------

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