[GHC] #10817: Looping default associated type family without UndecidableInstances

[GHC]

#10817: Looping default associated type family without UndecidableInstances
-------------------------------------+-------------------------------------
              Reporter:  goldfire    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 When I say

 {{{
 {-# LANGUAGE TypeFamilies #-}

 module Bug where

 import Data.Proxy

 class C a where
   type F a
   type F a = F a

 instance C Bool

 x :: Proxy (F Bool)
 x = Proxy
 }}}

 GHC just loops. Setting a low `-ftype-function-depth` doesn't help. I
 actually don't terribly mind the looping (although aborting with an
 overflow would be better). But I don't like that it's possible without
 `UndecidableInstances` in sight.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10817>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler