[GHC] #10675: GHC does not check the functional dependency consistency condition correctly

[GHC]

#10675: GHC does not check the functional dependency consistency condition
correctly
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.1
      Resolution:                    |             Keywords:  FunDeps
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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Comment (by diatchki):

 Hello,

 I just saw this, sorry for the very late reply.

 I don't think that the original example technically violates the FD
 property as the second instance (the one with `C Char ...`) is vacuous and
 doesn't actually add any new instances to `C` (there is no base case).
 The first instances on its own is OK, so the whole program is OK.  I am
 not sure that this is any easy thing to spot in general though.

 There is, however, a real problem with the consistency checking of FDs
 though, as illustrated by the following example:

 {{{
 class F tag a b | a -> b
 class G     a b | a -> b
 class H     a b | a -> b

 instance G a b => F Int  [a] [b]
 instance H a b => F Char [a] [b]
 instance G Int Int
 instance H Int Char
 }}}

 These instances are accepted by GHC 8.0.1 but are inconsistent, because we
 can derive both `F Int [Int] [Int]` and `F Char [Int] [Char]` which
 violates the FD on `F`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10675#comment:12>
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