https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67661
--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- You'll need to give a full testcase (complete compilable file and options used to compile it). What you gave isn't a compilable testcase; it gives "error: variably modified 'y' at file scope". Put inside a function, it gives "warning: unused variable 'y' [-Wunused-variable]", but does not give the warning you mention. And there's no variable 'b' in your example at all.
