Another dead head?!?

[Manuel M T Chakravarty]

validate dies for me with the appended error on MacOS with the 
latest HEAD.

Manuel

/opt/local/bin/ghc -Werror -H64m -Onot -fasm  -istage1/utils 
-istage1/basicTypes  -istage1/types  -istage1/hsSyn 
-istage1/prelude  -istage1/rename  -istage1/typecheck 
-istage1/deSugar  -istage1/coreSyn  -istage1/vectorise 
-istage1/specialise  -istage1/simplCore  -istage1/stranal 
-istage1/stgSyn  -istage1/simplStg  -istage1/codeGen  -istage1/main 
  -istage1/profiling  -istage1/parser  -istage1/cprAnalysis 
-istage1/ndpFlatten  -istage1/iface  -istage1/cmm 
-istage1/nativeGen -Wall -fno-warn-name-shadowing -fno-warn-orphans 
-Istage1 -cpp -fglasgow-exts -fno-generics -Rghc-timing -I. -Iparser 
-package unix -ignore-package lang -recomp -Rghc-timing -O -fasm 
-H16M '-#include "cutils.h"' -DUSING_COMPAT -i../compat 
-ignore-package Cabal    -c cmm/CmmZipUtil.hs -o 
stage1/cmm/CmmZipUtil.o  -ohi stage1/cmm/CmmZipUtil.hi

cmm/MkZipCfg.hs:330:44:
     Couldn't match expected type `m ()' against inferred type `()'
     In the first argument of `return', namely `()'
     In the expression: return ()
     In the definition of `note_this_code_becomes_unreachable':
         note_this_code_becomes_unreachable = return ()