[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
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------
Changes (by thomie):

 * component:  Compiler => Compiler (Type checker)


Comment:

 Here is a reduced testcase, without any dependencies.

 {{{#!hs
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeFamilies #-}

 module T12040 where

 import Data.Proxy


 -- src/Numeric/Units/Dimensional/Variant.hs

 data Variant = DQuantity


 -- src/Numeric/Units/Dimensional/Internal.hs

 class KnownVariant (v :: Variant) where
   data Dimensional v :: Dimension -> * -> *

 instance KnownVariant 'DQuantity where
   newtype Dimensional 'DQuantity d a = Quantity a

 type Quantity = Dimensional 'DQuantity


 -- ' -- src/Numeric/Units/Dimensional/Dimensions/TypeLevel.hs

 data Dimension = Dim -- TypeInt TypeInt TypeInt TypeInt TypeInt TypeInt
 TypeInt

 class HasDynamicDimension a where

 class HasDynamicDimension a => HasDimension a where

 type KnownDimension (d :: Dimension) = HasDimension (Proxy d)


 -- src/Numeric/Units/Dimensional/Dynamic.hs

 class Demotable (q :: * -> *) where

 instance (KnownDimension d) => Demotable (Quantity d) where
 }}}

 {{{
 $ ghc-7.10.3 T12040.hs -v0
 # ok

 $ ghc-8.0.1 T12040.hs -v0

 T12040.hs:45:10: error:
     • The constraint ‘KnownDimension d’
         is no smaller than the instance head
       (Use UndecidableInstances to permit this)
     • In the instance declaration for ‘Demotable (Quantity d)’
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12040#comment:2>
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