[GHC] #14046: “Illegal type synonym family application in instance” is too strict in the presence of functional dependencies

[GHC]

#14046: “Illegal type synonym family application in instance” is too strict in the
presence of functional dependencies
-------------------------------------+-------------------------------------
           Reporter:  lexi.lambda    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #3485
Differential Rev(s):                 |         Wiki Page:
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 GHC rejects the following instance, complaining about the use of a type
 synonym family in the instance head:

 {{{#!hs
 type family Fam a
 class Multi a b | a -> b
 instance Multi (Maybe a) (Fam a)
 }}}

 However, this seems too strict to me, since the second parameter of
 `Multi` is determined by a functional dependency. If I tweak the instance
 slightly to lift the second parameter out of the instance head into an
 equality constraint, GHC accepts the instance:

 {{{#!hs
 instance (b ~ Fam a) => Multi (Maybe a) b
 }}}

 It seems to me that, due to the functional dependency, these two instances
 are completely equivalent. Therefore, GHC should accept the first
 instance.

 This is similar to an old ticket, #3485, but that did not involve fundeps,
 so GHC’s error was correct. I’m pretty sure the addition of functional
 dependencies changes things, making the two actually equivalent.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14046>
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