[GHC] #13398: Associated type family instance validity checking is too conservative

[GHC]

#13398: Associated type family instance validity checking is too conservative
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11450            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:2 simonpj]:
 > Why did you not write (in the original) ...

 Because I didn't write this code. It's from Hackage.

 > But really, what's wrong with just using a variable here?

 Because that trick will only work in very limited circumstances. To better
 illustrate the point I'm trying to make, imagine you instead had this
 code:

 {{{#!hs
 instance HasStaticDuration (t :/ r) where
   type SetStaticDuration (t :/r) (t' :/ r') = r' :/ t'
 }}}

 This is a perfectly legitimate associated type family instance, and
 moreover, it //requires// pattern-matching on the second argument to
 `SetStaticDuration` to define. Yet in GHC HEAD, you cannot do this—you
 necessarily must jump through hoops to accomplish the same thing.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13398#comment:3>
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