[GHC] #12844: No Skolem Info with PartialTypeSignatures

[GHC]

#12844: No Skolem Info with PartialTypeSignatures
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           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  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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 The following program triggers a panic:

 {{{
 {-# LANGUAGE DataKinds             #-}
 {-# LANGUAGE PartialTypeSignatures #-}
 {-# LANGUAGE PolyKinds             #-}
 {-# LANGUAGE TypeFamilies          #-}
 {-# LANGUAGE TypeOperators         #-}

 barWraper :: ('(r,r') ~ Head rngs, Foo rngs) => FooData rngs
 barWraper = bar

 bar :: (_) => FooData rngs
 bar = foo

 data FooData rngs

 class Foo xs where foo :: (Head xs ~ '(r,r')) => FooData xs

 type family Head (xs :: [k]) where Head (x ': xs) = x
 }}}

 {{{
 > ghci NoSkolem.hs
 [1 of 1] Compiling Main             ( NoSkolem.hs, NoSkolem.o )

 NoSkolem.hs:8:13: error:ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-unknown-linux):
         No skolem info: k_aYV[sk]
 }}}

 I haven't tested with 8.0.2 or head.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12844>
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