[GHC] #15712: GHC panic with -XDerivingVia

[GHC]

#15712: GHC panic with -XDerivingVia
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:  DerivingVia
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Iceland_jack:

Old description:

> I slipped up and typed `Codensity GEndo` and now `Codensity (GEndo m)`,
> resulting in a GHC panic
>
> {{{#!hs
> {-# Language RankNTypes  #-}
> {-# Language DerivingVia #-}
>
> import Control.Monad.Codensity
> import Data.Kind
>
> newtype GEndo m a = GEndo (m a -> m a)
>
> newtype LogicT m a = LogicT { runLogicT :: forall xx. (a -> (m xx -> m
> xx)) -> (m xx -> m xx) }
>   deriving
>     (Functor, Applicative, Monad)
>   via
>     (Codensity GEndo)
> }}}
>
> {{{
> $ ghci -ignore-dot-ghci 476.hs
> GHCi, version 8.7.20180828: http://www.haskell.org/ghc/  :? for help
> [1 of 1] Compiling Main             ( 476.hs, interpreted )
> ghc-stage2: panic! (the 'impossible' happened)
>   (GHC version 8.7.20180828 for x86_64-unknown-linux):
>         ASSERT failed!
>   Type-correct unfilled coercion hole {co_a2aG}
>   Call stack:
>       CallStack (from HasCallStack):
>         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
> ghc:Outputable
>         pprPanic, called at compiler/utils/Outputable.hs:1219:5 in
> ghc:Outputable
>         assertPprPanic, called at compiler/typecheck/TcHsSyn.hs:1716:99
> in ghc:TcHsSyn
>
> Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
>
> >
> }}}
>
> Deriving via `Codensity (GEndo m)` works as expected.

New description:

 I slipped up and typed `Codensity GEndo` instead of `Codensity (GEndo m)`,
 resulting in a GHC panic

 {{{#!hs
 {-# Language RankNTypes  #-}
 {-# Language DerivingVia #-}

 import Control.Monad.Codensity
 import Data.Kind

 newtype GEndo m a = GEndo (m a -> m a)

 newtype LogicT m a = LogicT { runLogicT :: forall xx. (a -> (m xx -> m
 xx)) -> (m xx -> m xx) }
   deriving
     (Functor, Applicative, Monad)
   via
     (Codensity GEndo)
 }}}

 {{{
 $ ghci -ignore-dot-ghci 476.hs
 GHCi, version 8.7.20180828: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( 476.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20180828 for x86_64-unknown-linux):
         ASSERT failed!
   Type-correct unfilled coercion hole {co_a2aG}
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/utils/Outputable.hs:1219:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/typecheck/TcHsSyn.hs:1716:99 in
 ghc:TcHsSyn

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 >
 }}}

 Deriving via `Codensity (GEndo m)` works as expected.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15712#comment:1>
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