Invalid core generated with GHC 7.10 RC3

Using the Shake repo (https://github.com/ndmitchell/shake.git) at commit d7fa04 and GHC 7.10 RC3 or GHC HEAD, if I {{cabal test}} I get the error:

## BUILD oracle --quiet *str-int
TESTS FAILED
shake-test: Expected an exception but succeeded

Discussion about this issue can be found on the mailing list: http://osdir.com/ml/general/2015-03/msg25847.html

GHC generates the Core:

case (\_ -> error "here") of {}

This is invalid GHC Core. I intend to track down a minimal example shortly.

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