[GHC] #7066: isInstance does not work for compound types

[GHC]

#7066: isInstance does not work for compound types
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        Reporter:  edsko             |            Owner:  simonpj
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Template Haskell  |          Version:  7.4.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by mojojojo):

 I just got hit by this issue.

     I would expect
     {{{
     isInstance clss typs
     }}}
     to return True if and only if I can safely generate code (in my
 Template Haskell code) that relies on 'typs' being an instance of 'clss'.
 In the example above, I can not safely generate code that relies on a Show
 instance for (Int, A) because there is no Show instance for A.

 I expected exactly the same. That's why I believe that the current
 behaviour of the function is unintuitive to say the least. The
 documentation has no info concerning such behaviour either.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7066#comment:8>
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