[GHC] #15170: GHC HEAD panic (dischargeFmv)

[GHC]

#15170: GHC HEAD panic (dischargeFmv)
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 The patch in comment:3 makes the program in comment:1 compile fine.

 However, the program in the Description now fails with a Lint error:
 {{{
 *** Core Lint errors : in result of Desugar (before optimization) ***
 <no location info>: warning:
     In the type ‘forall (x :: a_a10V[sk:0]) a (b :: a
                                                     ~> *) (c :: forall (x
 :: a).
                                                                 Proxy x
                                                                 ~> ((b @@
 x)
                                                                     ~> *))
 (f :: forall (x :: a) (y :: b
 @@ x).
 Proxy x
 ~> (Proxy y
 ~> ((c x
 @@ 'Proxy)
 @@ y))) (g :: forall (x :: a).
 Proxy
 x
 ~> (b
 @@ x)) (x :: a).
                  SingKind ((c x @@ 'Proxy) @@ (g x @@ 'Proxy)) =>
                  (forall (xx :: a) (yy :: b @@ xx).
                   Sing ((f xx yy @@ 'Proxy) @@ 'Proxy))
                  -> Sing (g x)
                  -> Sing a
                  -> Demote ((c x @@ 'Proxy) @@ (g x @@ 'Proxy))’
     @ a_a10V[sk:0] is out of scope
 }}}
 I believe that the cause is the same as #14880, which I would love to get
 nailed.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15170#comment:4>
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