[GHC] #8651: 'Untouchable' error when using type function in class constraint in rank-2 type

[GHC]

#8651: 'Untouchable' error when using type function in class constraint in rank-2
type
-------------------------------------------+-------------------------------
       Reporter:  sbarclay                 |             Owner:
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.7
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:  #8644 #7594              |         Test Case:
                                           |          Blocking:
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 I noticed there are some cases that no longer compile after the recent fix
 for #8644, such as the following:

 {{{#!haskell
 {-# LANGUAGE RankNTypes, FlexibleContexts, TypeFamilies #-}

 import Data.Monoid

 type family Id a

 type instance Id a = a
 --type instance Id [a] = [Id a]

 foo :: (Monoid (Id String) => r) -> r
 foo x = x

 main :: IO ()
 main = print $ foo "Hello"
 }}}

 Attempting to compile this on HEAD produces the same error in 'main' as
 reported in the earlier ticket:

 {{{
     Couldn't match expected type ‛s0’ with actual type ‛[Char]’
       ‛s0’ is untouchable
 }}}

 However, it compiles fine if the commented-out type family instance is
 used instead. It also compiles fine if the type family is replaced with an
 ordinary type synonym:

 {{{#!haskell
 type Id a = a
 }}}

 I guess that the problem is caused by equalities of the form t ~ [Char]
 introduced by the type family instance.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8651>
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