[GHC] #16008: GHC HEAD type family regression involving invisible arguments

7 Dec 2018

      #16008: GHC HEAD type family regression involving invisible arguments
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.7
  (Type checker)                     |
           Keywords:  TypeFamilies,  |  Operating System:  Unknown/Multiple
  TypeInType                         |
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following code compiles on GHC 8.0.2 through 8.6.2:

 {{{#!hs
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 module Bug where

 import Data.Kind

 class C k where
   type S :: k -> Type

 data D :: Type -> Type
 data SD :: forall a. D a -> Type

 instance C (D a) where
   type S = SD
 }}}

 But fails to compile on GHC HEAD (commit
 73cce63f33ee80f5095085141df9313ac70d1cfa):

 {{{
 $ ~/Software/ghc2/inplace/bin/ghc-stage2 Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:15:3: error:
     • Type indexes must match class instance head
       Expected: S @(D a)
         Actual: S @(D a1)
     • In the type instance declaration for ‘S’
       In the instance declaration for ‘C (D a)’
    |
 15 |   type S = SD
    |   ^^^^^^^^^^^
 }}}

 This regression prevents
 [https://cs.brynmawr.edu/~rae/papers/2018/stitch/stitch.tar.gz the stitch
 library] from compiling.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16008
GHC http://www.haskell.org/ghc/
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