[GHC] #12731: Generic type class has type family; leads to big dep_finsts

[GHC]

#12731: Generic type class has type family; leads to big dep_finsts
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        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * keywords:   => Generics
 * cc: RyanGlScott (added)


Comment:

 As you've discovered, it's possible to manually define `Generic` instances
 (as long as Safe Haskell isn't enabled, I might point out). But even if we
 restrict `Generic` instances to only be derived, I don't think that would
 guarantee they don't overlap. After all, you can do this (requires GHC 8.0
 or later):

 {{{#!hs
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE StandaloneDeriving #-}

 import GHC.Generics

 data Foo a = Foo a
 deriving instance {-# OVERLAPPABLE #-} Generic (Foo a)
 deriving instance {-# OVERLAPPING  #-} Generic (Foo Int)
 }}}

 We used to forbid `Generic` instances like `Generic (Foo Int)` (see
 #5939), but reverted that decision when we found out that it wreaked havoc
 with sufficiently poly-kinded `Generic1` instances that require kinds to
 be instantiated with `*` (see #11732).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12731#comment:3>
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