[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
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------

Old description:

> 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
> }}}

New description:

 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
 }}}

--

Comment (by aaronvargo):

 Oops, it seems this only happened because I foolishly had `IllKindedA` in
 the same library as `A`:

 {{{
 library foo
   build-depends: base
   signatures: A
   exposed-modules: Foo, IllKindedA

 library bar
   build-depends: base, foo
   exposed-modules: Bar
   mixins: foo (Foo as Bug) requires (A as IllKindedA)
 }}}

 It's still a bug though.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15984#comment:1>
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