[GHC] #7585: Core lint failure when optimizing coercions in branched axioms

[GHC]

#7585: Core lint failure when optimizing coercions in branched axioms
-------------------------------+--------------------------------------------
Reporter:  goldfire            |          Owner:  goldfire        
    Type:  bug                 |         Status:  new             
Priority:  normal              |      Component:  Compiler        
 Version:  7.7                 |       Keywords:  TypeFamilies    
      Os:  Unknown/Multiple    |   Architecture:  Unknown/Multiple
 Failure:  Compile-time crash  |      Blockedby:                  
Blocking:                      |        Related:                  
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 The attached code causes the failure.

 Core Lint correctly checks branched axioms for internal consistency --
 when using branch ''n'' of an axiom, we must ensure that no branch ''m <
 n'' can possibly apply, no matter what the instantiation for any type
 variables in the branch may be. However, the coercion optimizer does not
 respect this property. It will replace coercions used in axioms with
 equivalent coercions that do not respect this internal consistency
 property. Everything works out OK in the end (without {{{-dcore-lint}}},
 the file compiles and runs correctly), but we go through an invalid state
 on the way.

 In particular, the {{{TrPushAx}}} rules are to blame.

 I'm not sure what the best fix for this is, but it seems to be my job to
 find it.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7585>
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