[GHC] #8129: Constraint solver panic due to derived type function equality (was: Constraint solver panic when -ddump-tc-trace is used)

[GHC]

#8129: Constraint solver panic due to derived type function equality
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        Reporter:  adamgundry               |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  high                     |        Milestone:
       Component:  Compiler (Type checker)  |          Version:  7.7
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Compile-time crash       |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Changes (by adamgundry):

 * priority:  normal => high


Comment:

 The above example, without `-ddump-tc-trace`, now causes the latest HEAD
 to panic with:
 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.7.20130915 for i386-unknown-linux):
         ASSERT failed!
     file compiler/typecheck/TcMType.lhs line 809
     [D] _ :: main:Main.F{tc rpc} x_aAu{tv} [tau[0]]
              ghc-prim:GHC.Types.~{(w) tc 31Q} y_aAv{tv} [tau[0]]
 (CFunEqCan)
 }}}
 It looks like the problem was introduced in
 e365d4963f1061878269502f256b4a56ca273a78 (the fix to #8262). The new code
 calls `solveWantedsTcMWithEvBinds` with `solve_wanteds`, rather than
 `solve_wanteds_and_drop`. But `solveWantedsTcMWithEvBinds` calls `zonkWC`,
 which works only with wanted (not derived) constraints.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8129#comment:1>
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