[GHC] #681: non-terminating instances involving FDs

[GHC]

#681: non-terminating instances involving FDs
-------------------------+--------------------------------------------------
    Reporter:  ross      |        Owner:         
        Type:  bug       |       Status:  new    
    Priority:  normal    |    Milestone:         
   Component:  Compiler  |      Version:  6.4.1  
    Severity:  normal    |     Keywords:         
          Os:  Unknown   |   Difficulty:  Unknown
Architecture:  Unknown   |  
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GHC accepts the following (tc109) with -fglasgow-exts:
 {{{
 class P a
 class R a b | b->a

 instance (P a,R a b) => P [b]
 }}}
 However as pointed out in the FD-CHR paper (example 15), adding the
 instance
 {{{
 instance R [[a]] [a]
 }}}
 will cause non-termination. Such instances (context contains variables not
 in the head, even if determined by variables in the head) should be
 forbidden without -fallow-undecidable-instances. (This is the second part
 of the Bound Variable Condition in the above paper). Similar cases are
 tc118 and tc125.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/681>
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