[GHC] #13068: GHC should not allow modules to define instances of abstract type classes

[GHC]

#13068: GHC should not allow modules to define instances of abstract type classes
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:  backpack       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 hs-boot files permit a type class to be given "abstractly", in which case
 any implementation of the type class is permissible. But it does not
 reject instances defined for such a class.

 {{{
 -- A.hs-boot
 module A where
 class C a

 -- B.hs
 module B where
 import {-# SOURCE #-} A
 instance C Int where

 -- A.hs
 module A where
 import B
 class C a where
     f :: a

 -- Main.hs
 import A
 main = print (f :: Int)
 }}}

 I get this when I build with `--make`:

 {{{
 ezyang at sabre:~$ ghc-head --make C.hs -fforce-recomp
 [1 of 4] Compiling A[boot]          ( A.hs-boot, A.o-boot )
 [2 of 4] Compiling B                ( B.hs, B.o )
 [3 of 4] Compiling A                ( A.hs, A.o )
 [4 of 4] Compiling Main             ( C.hs, C.o )
 Linking C ...
 ./B.o:(.data+0x0): undefined reference to `A_CZCC_con_info'
 collect2: error: ld returned 1 exit status
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13068>
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