http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60018
--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> --- The issue is that at -O0 conversion_warning gets (fn3(), 0) || 0 and doesn't call unsafe_conversion_p on it, while on -O fold_binary folds away the || 0 part and conversion_warning sees COMPOUND_EXPR, it then calls unsafe_conversion_p on it. I guess nothing for 4.9 now we're in stage4.
