[GHC] #3485: "Illegal type synonym family application in instance" error is unnecessary, should be removed

[GHC]

#3485: "Illegal type synonym family application in instance" error is unnecessary,
should be removed
-------------------------------------+--------------------------------------
 Reporter:  Ashley Yakeley           |          Owner:                  
     Type:  bug                      |         Status:  closed          
 Priority:  normal                   |      Milestone:                  
Component:  Compiler (Type checker)  |        Version:  6.10.4          
 Severity:  normal                   |     Resolution:  invalid         
 Keywords:                           |       Testcase:                  
       Os:  Unknown/Multiple         |   Architecture:  Unknown/Multiple
-------------------------------------+--------------------------------------
Changes (by chak):

  * status:  new => closed
  * resolution:  => invalid

Comment:

 I am not sure what you are suggesting.  We cannot automatically transform
 {{{
 instance C (Fam Int)
 }}}
 into
 {{{
 instance (Fam Int ~ famint) => C famint
 }}}

 This works if there is only one instance, but as soon as there are two
 such instances, they always overlap.

 Maybe you are suggesting that we should do it anyway and programmers
 should just take the implicit transformation into account.  I don't think
 that this is a good idea.  It's confusing for very little benefit (as you
 can always write the transformed instance yourself with little effort).

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3485#comment:1>
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