Ah, I missed the warning because I forge -O0 when I build Python.
https://wiki.ubuntu.com/ToolChain/CompilerFlags
Ubuntu adds -D_FORTIFY_SOURCE=2 flag by default.
The warnings can be seen with "-D_FORTIFY_SOURCE=2 -Og", but not with "-D_FORTIFY_SOURCE=2 -O3". Moreover, "-D_FORTIFY_SOURCE=2 -O0" complains that _FORTIFY_SOURCE requires to optimize the code.
It looks more like a false alarm because -D_FORTIFY_SOURCE=2 is incompatible with -Og.
Maybe we should force -D_FORTIFY_SOURCE=0 when we build Python in debug mode?