[GHC] #11563: fvProv falls into a hole (instead of detecting missing type argument) (regression from -rc1 to -rc2)

[GHC]

#11563: fvProv falls into a hole (instead of detecting missing type argument)
(regression from -rc1 to -rc2)
-------------------------------------+-------------------------------------
           Reporter:  j.waldmann     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc2
           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:
-------------------------------------+-------------------------------------
 This program has an error (T is missing an argument)
 {{{
 data T a ; class C t ; instance C s => C T
 }}}
 which makes 8-rc2 fall into a hole:
 {{{
 /opt/ghc/ghc-8.0.0.20160204/bin/ghc -c Bug.hs
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.0.20160204 for x86_64-unknown-linux):
         fvProv falls into a hole {ap6}
 }}}
 It is detected by 8-rc1:
 {{{
 /opt/ghc/ghc-8.0.0.20160111/bin/ghc -c Bug.hs

 Bug.hs:1:40: error:
     • Illegal instance declaration for ‘C T’
         (All instance types must be of the form (T a1 ... an)
          where a1 ... an are *distinct type variables*,
          and each type variable appears at most once in the instance head.
          Use FlexibleInstances if you want to disable this.)
     • In the instance declaration for ‘C T’

 Bug.hs:1:42: error:
     • Expecting one more argument to ‘T’
       Expected a type, but ‘T’ has kind ‘* -> *’
     • In the first argument of ‘C’, namely ‘T’
       In the instance declaration for ‘C T’
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11563>
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