[GHC] #13065: Prohibit user-defined Generic and Generic1 instances

[GHC]

#13065: Prohibit user-defined Generic and Generic1 instances
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           Reporter:  dfeuer         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  Generics       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 User-defined `Generic` and `Generic1` instances are problematic.

 === They are susceptible to breakage ===
 Some details of the classes may change between GHC versions, and indeed
 have done so in the past. User-defined instances are likely to break in
 the face of various such "internal" changes. This is one reason why
 `Data.Sequence`, for example, does not have a `Generic` instance.

 === They require potentially-expensive consistency checks ===
 GHC cannot assume that every type has at most one `Generic` and `Generic1`
 instance, so it needs to look for possible alternatives at instance
 resolution time. According to Simon (and maybe also Simon), this may be
 partly responsible for the performance regressions seen in Phab:D2899.

 === Downsides ===
 Prohibiting user-defined instances does have some costs. Suppose a type
 was originally defined concretely, exposing its constructors and a
 `Generic` instance. The implementer may decide later to make the type
 abstract, and export pattern synonyms to retain the same interface. But
 the `Generic` instance will either change or disappear. Someone relying on
 that instance could be in trouble. If the instance disappears, they'll be
 forced to write code by hand that they didn't need to before. If it
 changes, their code may change its behavior unexpectedly.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13065>
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