Weird error message ('Cannot determine version of GHC')

[Anthony Chaumas-Pellet]

Hello,

I've just run across a fairly confusing error message when running
'cabal install':

  Warning: cannot determine version of /usr/local/bin/ghc :
  ""
  cabal: ghc version >=6.2 is required but the version of
  /usr/local/bin/ghc could not be determined.

This error only appears with the install command, apparently when
Cabal is run from a directory where it cannot write files (I was under
/sys/, but the same goes for e.g. /usr/ as a non-root user).

If possible, I think the error message should be clearer (it took me a
while to figure out what was going on, and Google had no hits for that
error message).

I'm running under Debian Unstable, with ghc-6.8.1 and a very recent
Cabal Darcs build. Unfortunately, I cannot try reproducing that glitch
on the latest Cabal release.

Thanks!
Anthony