[GHC] #16181: ghc panic when using DerivingVia

[GHC]

#16181: ghc panic when using DerivingVia
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           Reporter:  chessai        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following program results in a ghc panic:

 {{{#!hs
 {-# LANGUAGE DerivingVia #-}

 import Data.Functor.Const (Const(..))
 import Data.Functor.Classes

 newtype FlipConst a b = FlipConst b
   deriving (Show1, Eq1) via (Const b)
 }}}

 Here is the output of the compile:

 {{{#!sh
 test/Spec/Contravariant.hs:52:13: error:
     • No instance for (Show b)
         arising from the 'deriving' clause of a data type declaration
       Possible fix:
         add (Show b) to the context of
           the deriving clause for ‘Show1 (FlipConst a)’
         or use a standalone 'deriving instance' declaration,
              so you can specify the instance context yourself
     • When deriving the instance for (Show1 (FlipConst a))
    |
 52 |   deriving (Show1, Eq1) via (Const b)
    |             ^^^^^

 test/Spec/Contravariant.hs:52:13: error:ghc: panic! (the 'impossible'
 happened)
   (GHC version 8.6.3 for x86_64-unknown-linux):
         No skolem info:
   [b_a3eO]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/TcErrors.hs:2891:5 in
 ghc:TcErrors

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

 }}}

 I'm using GHC 8.6.3

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16181>
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