[GHC] #9443: Regression from 7.6.3 to 7.8.3: could not coerce because argument "has role Nominal"

[GHC]

#9443: Regression from 7.6.3 to 7.8.3: could not coerce because argument "has role
Nominal"
-------------------------------------+-------------------------------------
       Reporter:  bernalex           |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  x86_64 (amd64)     |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  GHC
     Blocked By:                     |  rejects valid program
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 This file works fine with 7.6.3:
 <https://github.com/plaimi/plailude/blob/master/src/Data/Time/Units.hs>

 But with 7.8.3, every newtype has an error message like:
 {{{
 Could not coerce from ‘TimeUnit Integer’ to ‘TimeUnit (Year a)’
 ||   because the first type argument of ‘TimeUnit’ has role Nominal,
 ||   but the arguments ‘Integer’ and ‘Year a’ differ
 ||   arising from the coercion of the method ‘timeVal’ from type
 ||                ‘forall n. (TimeUnit Integer, Num n) => Integer -> n’ to
 type
 ||                ‘forall n. (TimeUnit (Year a), Num n) => Year a -> n’
 }}}

 To make the code work again, I need to remove a redundant type constraint
 from:
 {{{#!hs
 class TimeUnit t where timeVal :: (TimeUnit t, Num n) => t -> n
 }}}

 Turning it into:
 {{{#!hs
 class TimeUnit t where timeVal :: Num n => t -> n
 }}}

 The constraint is redundant because it is already implied by 'class
 TimeUnit t'.


 I suspect this is a regression. I would be happy to work on it, but I
 don't have time right now, so I'm putting it here for visibility,
 feedback, and to remember it in the first place.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9443>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler