The build breaks - applyTys

[Manuel M. T. Chakravarty]

After SimonPJ's recent commit of prelude/PrelRules.lhs, I
get 

/home/chak/sandbox/bin/ghc -DGHCI -cpp -fglasgow-exts -Rghc-timing -I. -IcodeGen -InativeGen -Iparser -iutils:basicTypes:types:hsSyn:prelude:rename:typecheck:deSugar:coreSyn:specialise:simplCore:stranal:stgSyn:simplStg:codeGen:absCSyn:main:profiling:parser:usageSP:cprAnalysis:javaGen:compMan:nativeGen:ghci  -package concurrent -package posix -package text -package util -recomp -O -Rghc-timing -H16m  -fvia-C  -c prelude/PrelRules.lhs -o prelude/PrelRules.o -osuf o
ghc: panic! (the `impossible' happened):
	applyTys

when trying to build GHCi with the HEAD.  More specifically,
`/home/chak/sandbox/bin/ghc' is the HEAD from the 13th
February including GHCi support. 

NB: The last time, I built GHCi with the old version of
prelude/PrelRules.lhs, I actually did so using exactly the
same version of GHC that fails now, but *without* GHCi
support.  (I don't have this build anymore, therefore, I
can't try it with that.)

Cheers,
Manuel