[GHC] #11331: panic! TEST=tc198: lookupVers2 GHC.Stack.Types CallStack

[GHC]

#11331: panic! TEST=tc198: lookupVers2 GHC.Stack.Types CallStack
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        Reporter:  thomie            |                Owner:  gridaphobe
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:
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 gridaphobe):

 A couple initial observations:

 1. The panic happens when ghc builds the ModIface, specifically when
 fingerprinting `GHC.Stack.Types.CallStack`.

 2. The panic only occurs with `-O`. The module is so simple that the Core
 is unchanged by optimizations, but the optimized Core does include an
 unfolding that mentions the `CallStack` type.

 So, it looks like GHC is trying to fingerprint `CallStack` to write out
 the interface file, but fails because `GHC.Stack.Types` was never loaded.
 The question then is why did we not load `GHC.Stack.Types`? It's clearly
 imported by `GHC.Err`, the interface file confirms. But `-ddump-if-trace`
 doesn't mention `GHC.Stack.Types`..

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11331#comment:2>
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