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This is enough to trigger a crash on OS X and Linux:

Bug1.hs:

module Bug1.hs where
import qualified Bug2

test :: IO Bool
test = Bug2.failure

Bug2.hs:

module Bug2 where

failure :: IO Bool
failure = return False

Shell:

% ghci -fdefer-type-errors -ignore-dot-ghci
GHCi, version 8.4.1: http://www.haskell.org/ghc/  :? for help
Prelude> :load Bug
[1 of 2] Compiling Bug2             ( Bug2.hs, interpreted )
[2 of 2] Compiling Bug              ( Bug.hs, interpreted )
Ok, two modules loaded.
*Bug> test
ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.1 for x86_64-apple-darwin):
	nameModule
  system $dShow_a1LX
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/basicTypes/Name.hs:241:3 in ghc:Name

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

This is specific to 8.4.1, in 8.0.2 I get "False" as expected. If I leave off -fdefer-type-errors, it works. It also seems to be ghci only, compiling with -fdefer-type-errors doesn't have the problem.

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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