[GHC] #12803: Functional dependencies and type families

[GHC]

#12803: Functional dependencies and type families
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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:                    |
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Description changed by simonpj:

@@ -17,0 +17,2 @@
+
+ I realised this when investigating #10778.

New description:

 Consider
 {{{
 type family F a :: *

 class C a b | a -> b

 instance C p (F q) => C p [q]
 }}}
 Is the instance decl OK under the liberal coverage condition?

 That is, does `(C t [s1], C t [s2])` mean that `(s1 ~ s2)`?

 NO!  The context of the instance declaration means that `p -> F q` but
 that does not imply `p -> q` unless `F` is injective!

 So this program is wrongly accepted.

 I realised this when investigating #10778.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12803#comment:1>
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