Created attachment 54071 [details]
Reproducer

Compiling with -O2 or higher I get an incorrect warning:
test.c: In function ‘foo3’:                                                                                             
test.c:15:5: warning: ‘foo’ accessing 16 bytes in a region of size 8 [-Wstringop-overflow=]                             
 
This doesn't appear at lower optimization levels.  It's also very sensative to the function, for example removing the return on the next line makes the warning go away.