http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53920
--- Comment #1 from Samuel Bronson <naesten at gmail dot com> 2012-07-10 19:07:26 UTC --- Oh, I suppose I should mention that I ran into this because I was using ccache to compile Emacs with --enable-gcc-warnings, and by default ccache runs the preprocessor and the compiler in separate passes (so that it can skip the compilation proper if it has cached output for a given preprocessor output), and then pastes together the diagnostic output from the two passes. (Thankfully, setting the environment variable CCACHE_CPP2 causes ccache to throw out the preprocessor output after hashing, which is a quite effective workaround for this sort of thing, though preprocessing the source twice does make things a bit slower on cache misses.)
