Using "ITstring !FastString" in Lexer.x goes wrong (internal error: stg_ap_v_ret)

[igloo]

With this patch:

hunk ./compiler/parser/Lexer.x 535
+#if __GLASGOW_HASKELL__ >= 609
+  | ITstring     !FastString
+#else
hunk ./compiler/parser/Lexer.x 539
+#endif

something goes wrong. The first sign, when validating, is that timeout is broken:

timeout: internal error: stg_ap_v_ret
    (GHC version 6.9.20080902 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

If you use the python timeout program instead then you find a few tests fail, e.g.:

testsuite/tests/ghc-regress/codeGen/should_run$ '/home/ian/ghc/darcs/st2/ghc/stage2-inplace/ghc' -fforce-recomp -dcore-lint -dcmm-lint -Dx86_64_unknown_linux  -dno-debug-output -o cg052 cg052.hs -O -fasm -funbox-strict-fields
testsuite/tests/ghc-regress/codeGen/should_run$ ./cg052 
ok
cg052: internal error: stg_ap_v_ret
    (GHC version 6.9.20080902 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
zsh: abort      ./cg052

I'm not sure what's going on, but it seems to be repeatable.

[igloo]

I can't reproduce this in the HEAD.