[GHC] #8129: Constraint solver panic when -ddump-tc-trace is used

13 Aug 2013

      #8129: Constraint solver panic when -ddump-tc-trace is used
-------------------------------------+-------------------------------------
       Reporter:  adamgundry         |             Owner:
           Type:  bug                |            Status:  new
       Priority:  normal             |         Milestone:
      Component:  Compiler (Type     |           Version:  7.7
  checker)                           |  Operating System:  Unknown/Multiple
       Keywords:                     |   Type of failure:  Compile-time
   Architecture:  Unknown/Multiple   |  crash
     Difficulty:  Unknown            |         Test Case:
     Blocked By:                     |          Blocking:
Related Tickets:                     |
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 {{{
 {-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-}
 {-# OPTIONS_GHC -ddump-tc-trace #-}

 type family F (x :: *) :: *

 class (y ~ F x) => C x y

 z = () :: C x y => ()
 }}}

 causes a panic while tracing the typechecker:

 {{{
 solveNestedImplications starting {
   original inerts =  Equalities:
                      Type-function equalities: [D] _ :: F x_aeH ~ y_aeI
 (CFunEqCan)
                      Dictionaries: [W] $dC_aeK :: C x_aeH y_aeI (CDictCan)
                      Irreds:
                      Insolubles = {}
                      Solved dicts 0
                      Solved funeqs 0
   thinner_inerts  =  Equalities:ghc-stage2: panic! (the 'impossible'
 happened)
   (GHC version 7.7.20130812 for i386-unknown-linux):
         No match in record selector ctev_evar
 }}}

 If `-ddump-tc-trace` is not used, the correct constraint solving error is
 generated:

 {{{
     Could not deduce (C x0 y0)
       arising from the ambiguity check for an expression type signature
     from the context (C x y)
       bound by an expression type signature: C x y => ()
 }}}

 It seems that `prepareInertsForImplications` in `TcSMonad` assumes that
 all the
 `inert_funeqs` are givens or wanteds; if there are some deriveds, then the
 above panic results.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8129
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