[GHC] #8264: Canonical way to prevent manual instances

[GHC]

#8264: Canonical way to prevent manual instances
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        Reporter:  nomeata                  |            Owner:
            Type:  task                     |           Status:  closed
        Priority:  normal                   |        Milestone:
       Component:  Compiler (Type checker)  |          Version:  7.6.3
      Resolution:  fixed                    |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Changes (by nomeata):

 * status:  new => closed
 * resolution:   => fixed


Comment:

 This was introduced in 17a868afa169c52d8525a95cbed87b2fc12044c6,
 (`abstractClasses`, used in `checkValidInstHead`) for `Coercible` only so
 far. This position is, however, unsuitable for classes that are being
 derived, as these also have to pass through `checkValidInstHead`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8264#comment:3>
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