[GHC] #15870: No skolem info panic

[GHC]

#15870: No skolem info panic
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        Reporter:  sheaf             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.3
       Component:  Compiler (Type    |              Version:  8.6.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I know why the crash happens.

 * In the instance decl we bring into scope the skolems of the instance

 * Then, we process the local family instance equation.  In doing so we
 call the dreaded `tcFamTyPats` -- and it calls `solveEqualities`  (wrongly
 I believe)

 * This `solveEqualities` tries to report an error; but it can't see the
 skolems, hence the panic.

 Really `solveEqualities` should only be called in an empty type
 environment.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15870#comment:3>
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