[GHC] #8356: Strangeness with FunDeps

[GHC]

#8356: Strangeness with FunDeps
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        Reporter:  ksf                           |            Owner:
            Type:  bug                           |           Status:
        Priority:  normal                        |  closed
       Component:  Compiler                      |        Milestone:
      Resolution:  fixed                         |          Version:  7.7
Operating System:  Unknown/Multiple              |         Keywords:
 Type of failure:  GHC rejects valid program     |     Architecture:
       Test Case:                                |  Unknown/Multiple
  typecheck/should_fail/tcfail170                |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:
                                                 |  Related Tickets:
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Comment (by monoidal):

 The testcase `tcfail170` triggers a `pprTrace` in `FunDeps.lhs`. Is this
 intended?

 {{{
 $ ghc-7.7.20131027 tcfail170
 [1 of 1] Compiling ShouldFail       ( tcfail170.hs, tcfail170.o )
 cic main:ShouldFail.C{tc roB} [[p{tv aoE} [sk]], [q{tv aoF} [sk]]]
     ([a{tv aoC} [tv]], [b{tv aoD} [tv]])
     [(aoE, p{tv aoE} [sk])]
     [(aoF, q{tv aoF} [sk])]
     [(aoE, p{tv aoE} [sk])]
     []

 tcfail170.hs:7:10:
     Illegal instance declaration for ‛C [p] [q]’
       The coverage condition fails in class ‛C’
         for functional dependency: ‛a -> b’
       Reason: lhs type ‛[p]’ does not determine rhs type ‛[q]’
     In the instance declaration for ‛C [p] [q]’
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8356#comment:7>
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