[GHC] #14668: Ordering of declarations can cause typechecking to fail

13 Jan 2018

      #14668: Ordering of declarations can cause typechecking to fail
-------------------------------------+-------------------------------------
           Reporter:  heptahedron    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following will successfully typecheck:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeFamilies #-}

 data CInst

 data G (b :: ()) = G

 class C a where
   type family F a

 class (C a) => C' a where
   type family F' a (b :: F a)

 -- data CInst

 instance C CInst where
   type F CInst = ()

 instance C' CInst where
 type F' CInst (b :: F CInst) = G b
 }}}

 But if the `data CInst` declaration is moved to where it is currently
 commented out, typechecking fails with this error:

 {{{
 Test.hs:23:18: error:
     • Expected kind ‘F CInst’, but ‘b’ has kind ‘()’
     • In the second argument of ‘F'’, namely ‘(b :: F CInst)’
       In the type instance declaration for ‘F'’
       In the instance declaration for ‘C' CInst’
    |
 23 |   type F' CInst (b :: F CInst) = G b
    |
 }}}

 However, the data declaration //can// be in the lower position if the kind
 annotation for its argument is instead written as `data G (b :: F CInst) =
 G`.

 This behavior is also exhibited when G is a type family (I believe the
 sort of type family does not matter, but I know for sure closed and open
 type families).

 I was using GHC 8.2.2 when I discovered this, but user `erisco` on
 `#haskell` confirmed for 8.2.1 as well.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14668
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