[GHC] #7354: Panic with recursion-schemes package and unit

[GHC]

#7354: Panic with recursion-schemes package and unit
-------------------------------+--------------------------------------------
    Reporter:  amplitwist      |       Owner:                              
        Type:  bug             |      Status:  new                         
    Priority:  normal          |   Milestone:                              
   Component:  GHCi            |     Version:  7.6.1                       
    Keywords:                  |          Os:  Linux                       
Architecture:  x86_64 (amd64)  |     Failure:  GHCi crash                  
  Difficulty:  Unknown         |    Testcase:  indexed_types/T7354a, T7354b
   Blockedby:                  |    Blocking:                              
     Related:                  |  
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Comment(by shachaf):

 For what it's worth, elliott in #haskell came across this bug in a
 different context. Here are two simpler cases (7.6.1):

 * Panics:

 {{{
 type family T :: * -> *
 x :: ()
 x = undefined :: T a
 }}}

 * Doesn't panic, but type-checks when it shouldn't:

 {{{
 type family T :: * -> *
 x :: ()
 x = undefined :: (b ~ T a) => b
 }}}

 Note that if you make an instance of T (e.g. `newtype Id a = Id a; type
 instance T = Id`), then even uses of Id unrelated to `x` cause a panic, as
 long as `x` exists.

 `-dcore-lint` catches the first case before it gets to a "real" panic.
 None of this is problematic in HEAD.

 Note: I haven't managed to get `unsafeCoerce` out of this, but that
 doesn't mean it's impossible. It's important to keep in mind that in the
 presence of SafeHaskell, type checker bugs can have security implications.
 Maybe `-XSafe` should imply/suggest `-dcore-lint`, or something along
 those lines? That's come up in a few other places, like #7453. Using
 SafeHaskell safely should be as easy as possible, not rely on obscure
 folklore and "compiler debugging options".

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7354#comment:7>
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