[GHC] #13262: Allow type synonym family application in instance head if it has no free variables

[GHC]

#13262: Allow type synonym family application in instance head if it has no free
variables
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  backpack
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  12680
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Artyom.Kazak):

 > Oh all right. We can wait till someone else pipes in and requests this
 :)

 That would be me!

 My usecase is generating anonymous records and then defining SafeCopy
 migrations for them (see
 http://hackage.haskell.org/package/safecopy-0.9.4.1/docs/Data-
 SafeCopy.html#t:Migrate). Essentially this:

 {{{
 type Foo_v1 = ...
 type Foo_v2 = AddField "name" Text Foo_v1
 type Foo_v3 = RemoveField "age" Foo_v2
 }}}

 All of that is implemented via type families. The problem is that now I
 can't have instances for these types (e.g. `instance Migrate Foo_v3`).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13262#comment:13>
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