[GHC] #12810: -Wredundant-constraints doesn't factor in associated type families

5 Nov 2016

      #12810: -Wredundant-constraints doesn't factor in associated type families
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           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           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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 If I compile this code:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module M where

 class C a where
   type T a

 instance C a => C [a] where
   type T [a] = T a
 }}}

 with `-Wredundant-constraints` enabled, it complains:

 {{{
 $ /opt/ghc/head/bin/ghc -Wredundant-constraints M.hs
 [1 of 1] Compiling M                ( M.hs, M.o )

 M.hs:7:10: warning: [-Wredundant-constraints]
     • Redundant constraint: C a
     • In the instance declaration for ‘C [a]’
 }}}

 I don't think this is right. The RHS of `T [a]` won't be able to reduce
 unless there's a `T a` instance available–that is, unless there's a `C a`
 instance available, which is what the context provides, making it non-
 redundant.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12810
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