[GHC] #9267: Lack of type information in GHC error messages when the liberage coverage condition is unsatisfied

[GHC]

#9267: Lack of type information in GHC error messages when the liberage coverage
condition is unsatisfied
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              Reporter:  danilo2     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  high        |         Milestone:  7.10.1
             Component:  Compiler    |           Version:  7.8.2
            Resolution:              |          Keywords:
Differential Revisions:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
            Difficulty:  Unknown     |         Test Case:
            Blocked By:              |          Blocking:
       Related Tickets:              |
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Comment (by simonpj):

 Interesting example.  See all the discussion on #8634.

 The interesting thing here is that (using the vocabulary of #8634)
  * The initial error message might suggest `-XDysFunctionalDepenencies`
  * Adding `-XDysFunctionalDependencies` would yield the error in GHC 7.6
  * Adding the suggested context to the instance declaration would remove
 the need for `-XDysFunctionalDependencies` again!

 If we implement the `DYSFUNCTIONAL` thing on a per-instance basis, as
 #8634 suggests, we should also warn when it is used but is not necessary.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9267#comment:4>
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