[GHC] #13850: Incorrect function signature causes the impossible to happen

[GHC]

#13850: Incorrect function signature causes the impossible to happen
-------------------------------------+-------------------------------------
           Reporter:  erikd          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:                 |  Operating System:  Linux
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I made a mistake in my function signature:
 {{{
 > cat wibble.hs
 ecase :: Either a b -> (a -> c) (b -> c) -> c
 ecase (Left a) f _ = f a
 ecase (Right b) _ g = g b

 > ghci wibble.hs
 GHCi, version 8.2.0.20170507: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/erikd/.ghci
 [1 of 1] Compiling Main             ( wibble.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.0.20170507 for x86_64-unknown-linux): repSplitAppTys
  a_a1pA[sk:1]
  c_a1pC[sk:1]
  []
  Call stack:
   CallStack (from HasCallStack):
    prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1134:58
 in
          ghc:Outputable
    callStackDoc, called at compiler/utils/Outputable.hs:1138:37 in
          ghc:Outputable
    pprPanic, called at compiler/types/Type.hs:808:9 in ghc:Type
 }}}

 GHC 8.02 correctly reports an error:
 {{{
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/erikd/.ghci
 [1 of 1] Compiling Main             ( wibble.hs, interpreted )

 wibble.hs:1:24: error:
     • Expecting one fewer argument to ‘a -> c’
       Expected kind ‘* -> *’, but ‘a -> c’ has kind ‘*’
     • In the type signature:
         ecase :: Either a b -> (a -> c) (b -> c) -> c
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13850>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler