[GHC] #11732: Deriving Generic1 interacts poorly with TypeInType

[GHC]

#11732: Deriving Generic1 interacts poorly with TypeInType
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        Reporter:  goldfire          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:  TypeInType,
                                     |  Generics
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5939             |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * related:   => #5939


Comment:

 OK, it sounds like we're converging on a consensus of being ultra-
 permissive in allowing GHC to unify visible type parameters when deriving
 instances.

 With that in mind, here's what we still need to do to fix this ticket:

 1. When generating a `Rep`/`Rep1` instance
 [http://git.haskell.org/ghc.git/blob/eb6b7094c80fda5cc7c1d1ed3386486996f24bff:/compiler/typecheck/TcGenGenerics.hs#l482
 here], do the appropriate type variable substitutions depending on the
 types the user provides.
 2. Remove the broken instantiation check
 [http://git.haskell.org/ghc.git/blob/eb6b7094c80fda5cc7c1d1ed3386486996f24bff:/compiler/typecheck/TcGenGenerics.hs#l155
 here].

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11732#comment:16>
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