[GHC] #8827: Inferring Safe mode with GeneralizedNewtypeDeriving is wrong

[GHC]

#8827: Inferring Safe mode with GeneralizedNewtypeDeriving is wrong
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       Reporter:  goldfire          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:  7.8.1
      Component:  Compiler          |           Version:  7.8.1-rc1
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Consider the following modules:

 {{{
 module A (List, ints) where

 data List a = Nil | Cons a (List a)
 infixr 4 `Cons`

 ints :: List Int
 ints = 1 `Cons` 2 `Cons` 3 `Cons` Nil
 }}}

 {{{
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}

 module B where

 import A

 newtype Age = MkAge Int
   deriving C

 class C a where
   cList :: List a

 instance C Int where
   cList = ints
 }}}

 {{{
 {-# LANGUAGE Safe #-}

 module C (ages) where

 import A
 import B

 ages :: List Age
 ages = cList
 }}}

 Module C compiles without a hiccup. But, it shouldn't: the coercion
 between `ages` and `ints` (performed by !GeneralizedNewtypeDeriving in
 module B) isn't Safe, as it breaks through `List`'s abstraction. (Note
 that the constructors of `List` are ''not'' exported from module A!)

 If module B includes `{-# LANGUAGE Safe #-}`, it duly doesn't compile,
 because of the stringent "constructors-must-be-in-scope" check done by the
 `Coercible` mechanism. The problem is that safety can be ''inferred''
 without this check taking place.

 You may also want to read the commentary on #8745 for related discussion.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8827>
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