[GHC] #14633: -fwarn-redundant-constraints false positive

3 Jan 2018

      #14633: -fwarn-redundant-constraints false positive
-------------------------------------+-------------------------------------
           Reporter:  ghorn          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I had code which compiled cleanly on GHC 8.0.2 with -fwarn-redundant-
 constraints which now gives a warning on GHC 8.2.2.

 Here is the code, and my workaround:

 {{{#!haskell
 {-# OPTIONS_GHC -Wall -Werror -fwarn-redundant-constraints #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE FlexibleContexts #-}

 module Bug
        ( bug
        , workaround
        ) where

 import GHC.Generics ( D1, Datatype, Meta, Rep, datatypeName )

 import Data.Proxy ( Proxy )

 -- /home/greghorn/hslibs/ghc82_bug_maybe/Bug.hs:17:1: warning:
 [-Wredundant-constraints]
 --     • Redundant constraint: Rep a ~ D1 d p
 --     • In the type signature for:
 --            bug :: forall a (d :: Meta) (p :: * -> *).
 --                   (Datatype d, Rep a ~ D1 d p) =>
 --                   Proxy a -> String
 --    |
 -- 25 | bug :: forall a d p . (Datatype d, Rep a ~ D1 d p) => Proxy a ->
 String
 --    |
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 bug :: forall a d p . (Datatype d, Rep a ~ D1 d p) => Proxy a -> String
 bug = const name
   where
     name = datatypeName (undefined :: D1 d p b)

 type family GetD a :: Meta where
   GetD (D1 d p) = d

 workaround :: forall a d p . (Datatype (GetD (Rep a)), Rep a ~ D1 d p) =>
 Proxy a -> String
 workaround = const name
   where
     name = datatypeName (undefined :: D1 d p b)
 }}}

 I suspect it is a bug because if I remove the "redundant" constraint it no
 longer typechecks.

 Here is a minimal setup to reproduce with stack:
 {{{
 name:                bug
 version:             0.0.0.2
 license:             AllRightsReserved
 author:              Greg Horn
 maintainer:          gregmainland@gmail.com
 build-type:          Simple
 cabal-version:       >=1.10

 library
   exposed-modules:     Bug
   build-depends:       base >= 4.7 && < 5
   default-language:    Haskell2010
 }}}

 {{{
 resolver: lts-10.2

 compiler-check: newer-minor

 # Local packages, usually specified by relative directory name
 packages:
 - .
 }}}

 Alternatively:
 {{{
 git clone https://github.com/ghorn/ghc-redundant-constraint-bug
 cd ghc-redundant-constraint-bug
 stack build
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14633
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