[GHC] #12522: GHC 8.0.1 hangs, looping forever in type-checker

[GHC]

#12522: GHC 8.0.1 hangs, looping forever in type-checker
-------------------------------------+-------------------------------------
        Reporter:  clinton           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"b255ae7b555b4b63085a6de4a7a6bd742326b9c9/ghc"
 b255ae7b/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="b255ae7b555b4b63085a6de4a7a6bd742326b9c9"
 Orient improvement constraints better

 This patch fixes an infinite loop in the constraint solver,
 shown up by Trac #12522.

 The solution is /very/ simple: just reverse the orientation of the
 derived constraints arising from improvement using type-family
 injectivity.  I'm not very proud of the fix --- it seems fragile
 --- but it has the very great merit of simplicity, and it works
 fine.

 See Note [Improvement orientation] in TcInteract, and some
 discussion on the Trac ticket.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12522#comment:16>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler