[GHC] #15984: Backpack accepts ill-kinded instantiations. Can cause GHC panic

[GHC]

#15984: Backpack accepts ill-kinded instantiations. Can cause GHC panic
-------------------------------------+-------------------------------------
           Reporter:  aaronvargo     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:  backpack       |  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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 Given the following:

 {{{#!hs
 {-# language KindSignatures #-}
 signature A where

 data A :: *
 }}}

 {{{#!hs
 module Foo where

 import A

 foo :: A -> A
 foo = id
 }}}

 {{{#!hs
 module IllKindedA where

 type A = Maybe
 }}}

 GHC allows the signature `A` to be instantiated with `IllKindedA`:

 {{{
 mixins: foo (Foo as Bug) requires (A as IllKindedA)
 }}}

 Using the resulting module can cause odd errors or a panic. E.g. the
 following causes a panic:

 {{{#!hs
 module Bar where

 import Bug

 bar = foo
 }}}

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.6.2 for x86_64-unknown-linux):
         getRuntimeRep
   A :: * -> *
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:2049:18 in ghc:Type
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15984>
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