[GHC] #7332: Kind-defaulting omitted leads to deeply obscure type error

[GHC]

#7332: Kind-defaulting omitted leads to deeply obscure type error
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    Reporter:  simonpj           |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  high              |   Milestone:  7.8.1           
   Component:  Compiler          |     Version:  7.6.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Changes (by simonpj):

  * priority:  normal => high
  * milestone:  => 7.8.1


Comment:

 Good catch!  In the transition from 7.4 I simplified the way that let-
 generalised types are inferred, and in so doing introduced an error.  In
 this case it's because we infer a constraint `Build (DC d) (a -> t)`,
 where `a :: OpenKind`.... and that means it doesn't match the instance
 declaration.  However it is defaulted to `a : *` during quantification,
 and the result is very confusing and wrong.

 Happily it's easy to fix.  Patch coming.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7332#comment:1>
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