[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 simonpj):

 Why did you not write (in the original)
 {{{
 instance HasStaticDuration (t :/ r) where
   type SetStaticDuration (t :/ r) pt = pt
 }}}
 I suppose you'll say that since `StaticTicks` has only one data
 constructor, replacing `pt` by `(t' :/ r')` makes no difference.  But what
 about
 {{{
 type family F a :: StaticTicks
 }}}
 Now I suppose that `SetStaticDuration (t :/ r) (F Int)` will fail to
 reduce?

 Suppose `StaticTicks` had a second data constructor `Foo`.  Would you
 still say that the instance should be accepted?

 I was just trying to enforce that the definition was complete, nothing
 deeper. There is nothing fundamentally wrong with a partial function I
 suppose.

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

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13398#comment:2>
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