[GHC] #12040: Code that builds on 7.8.4 and 7.10.3 but fails with requiring UndecidableInstances on 8.0.1-rc4

[GHC]

#12040: Code that builds on 7.8.4 and 7.10.3 but fails with requiring
UndecidableInstances on 8.0.1-rc4
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        Reporter:  dmcclean          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 It has something to do with the polykindedness of `Proxy`. This further-
 reduced version displays the same behavior but after replacing `Proxy` by
 `Maybe`, both versions of ghc accept the program.
 {{{
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE KindSignatures #-}

 module T12040 where

 import Data.Proxy

 newtype Dimensional c d a = Quantity a

 class HasDimension a
 class Demotable (q :: * -> *)

 instance (HasDimension (Proxy d)) => Demotable (Dimensional Int d) where
 }}}

 Maybe a consequence of kind and type arguments being treated uniformly in
 GHC's Core language now? Arguably neither behavior is wrong, since the
 instance termination checker is necessarily a conservative approximation.
 There's no real harm in just turning on UndecidableInstances.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12040#comment:4>
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