[GHC] #7438: Tc panic when type declaration is missing

[GHC]

#7438: Tc panic when type declaration is missing
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Reporter:  heisenbug         |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.7               |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  GHCi crash        |      Blockedby:                  
Blocking:                    |        Related:                  
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 Compile the attached snippet with HEAD GHC.

 > ghc: panic! (the 'impossible' happened)
 >   (GHC version 7.7.20121114 for x86_64-unknown-linux):
 >         tcTyVarDetails
 > <<details unavailable>>

 You need to install the "thrist-0.3" package from Hackage.

 When I just paste the 'Thrist' definition into the code the panic does not
 happen. (It resulted in this message for me:
 >     Occurs check: cannot construct the infinite type: t2 ~ 'St t2
 >     Expected type: Thrist Nat (At t4) t2 t3
 >       Actual type: Thrist Nat (At t4) b t3
 >     Relevant bindings include
 >       go :: Thrist Nat (At t4) t2 t3 -> t -> t1
 >         (bound at GHC-7438.hs:20:11)
 >     In the first argument of `go', namely `rest'
 >     In the second argument of `(:)', namely `go rest acc'
 >     In the expression: c : go rest acc
 > Failed, modules loaded: none.
 )

 Looks like interaction of non-generalized (local) let in conjunction with
 imported polykinded GADT.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7438>
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