[GHC] #11357: Regression when deriving Generic1 on poly-kinded data family

[GHC]

#11357: Regression when deriving Generic1 on poly-kinded data family
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.11
  (CodeGen)                          |             Keywords:  Generics,
      Resolution:                    |  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Something fishy is going on with the kind of the `ProxyFam` instance
 `TyCon`. I added `pprTrace`-ed the result of `tyConKind tc` right before
 the call to `filterOutInvisibleTypes` mentioned above, and noticed a
 discrepancy between datatypes and data families:

 {{{
 $ inplace/bin/ghc-stage2 --interactive
 GHCi, version 8.1.20160109: http://www.haskell.org/ghc/  :? for help
 λ> :set -XDeriveGeneric -XTypeFamilies -XPolyKinds -fprint-explicit-
 foralls
 λ> import GHC.Generics
 λ> data Proxy (a :: k) = Proxy deriving Generic1
 tc: Proxy
 tyConKind tc: forall k_a1Gj. k_a1Gj -> *
 tc_args: [*]
 filterOutInvisibleTypes tc tc_args: []
 all isTyVarTy: True
 λ> data family ProxyFam (a :: k)
 λ> data instance ProxyFam (a :: k) = ProxyCon deriving Generic1
 tc: R:ProxyFamka
 tyConKind tc: forall k_a1Ps -> k_a1Ps -> *
 tc_args: [*]
 filterOutInvisibleTypes tc tc_args: [*]
 all isTyVarTy: False

 <interactive>:5:53: error:
     • Can't make a derived instance of ‘Generic1 ProxyFam’:
         ProxyFam must not be instantiated; try deriving `ProxyFam k a'
 instead
     • In the data instance declaration for ‘ProxyFam’
 }}}

 Notice that `tyConKind` for the datatype `Proxy` is `forall k. k -> *`
 (i.e., `k` is specified but not visible), but the `tyConKind` for the
 `ProxyFam` instance is `forall k -> k -> *` (`k` is visible)! This is
 definitely the reason why this bug is manifesting itself, although I'm not
 sure why `k` is a visible type argument in the `ProxyFam` instance.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11357#comment:3>
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