[GHC] #13262: Allow type synonym family application in instance head if it reduces away

[GHC]

#13262: Allow type synonym family application in instance head if it reduces away
-------------------------------------+-------------------------------------
        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by ezyang):

 By analogy, though, `TypeSynonymInstances` are also distasteful, because
 it's effectively letting you write `myfalse = False; f myfalse = 5` (well,
 that doesn't actually do what you want in Haskell today :o)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13262#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler