I ran into a panic when updating singletons for 7.10. I'm clueless as to what's going on here, so sorry for not minimizing the test case. A little testing has me convinced it's Template Haskell in some way.

To reproduce:

> git clone http://github.com/goldfirere/singletons.git
> cd singletons
> git checkout ghc-loading-panic-test-case
> cabal update
> cabal install --only-dependencies
> cabal configure
> cabal build
> cat dist/build/autogen/cabal_macros.h
# copy the value for CURRENT_PACKAGE_KEY from the end of cabal_macros.h
> cd tests/compile-and-dump
> ghc -c -this-package-key <package key from cabal_macros.h> -i../../dist/build -XTemplateHaskell Singletons/Maybe.hs

You will see something like

ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.0.20150123 for x86_64-apple-darwin):
	Loading temp shared object failed: dlopen(/var/folders/ps/s45r2x1s6r15ws78py_zypl00000gn/T/ghc45837_0/libghc45837_1.dylib, 5): Symbol not found: _mtlzuJNaGzzEkFfL43R3LZZNRlPRm_ControlziMonadziReaderziClass_DZCMonadReader_con_info
  Referenced from: /var/folders/ps/s45r2x1s6r15ws78py_zypl00000gn/T/ghc45837_0/libghc45837_1.dylib
  Expected in: flat namespace
 in /var/folders/ps/s45r2x1s6r15ws78py_zypl00000gn/T/ghc45837_0/libghc45837_1.dylib

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

I've observed this on a Mac, but Travis has the same problem, so it's not strictly Mac-specific. You can see representative Travis output here.

Why am I doing such a crazy thing? It's part of the singletons test suite, where it's important to test the output of a run of ghc with -ddump-splices. Getting the test cases to compile against the built-but-not-yet-installed singletons object files should work with -this-package-key. I'm sure there's a better way to structure a testsuite, but this general technique works (with -package-name instead of -this-package-key) with 7.8.