[GHC] #11456: Type application and :set +c command cause panic

[GHC]

#11456: Type application and :set +c command cause panic
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications GHCi
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 It does actually, assuming you mean replacing `show` by `id` in the
 ticket.

 `GHCi.UI.Info.processAllTypeCheckedModule` is trying to desugar all the
 spans in the typechecked program individually to find out their types. One
 of those spans is `@Int`, which can't be desugared outside of the context
 `show @Int`. We should skip such subexpressions: any `HsTypeOut`
 constructor.

 Since this code is written in Generics-ese, I don't feel like trying to
 fix it myself.

 (Also, surely there is a more sensible way to collect this data then
 desugaring every span to find out its type, when we have already type
 checked the whole module?)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11456#comment:2>
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