https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70143
--- Comment #11 from Pedro Alves <palves at redhat dot com> --- > That is certainly not fine from aliasing perspective, aliasing is not just > about the type of the field you access, but the whole access path, so if you > use ((struct b *)ip)->j then ip should point to an object with effective type > of struct b. Sorry, bad choice of words (and I'm not that familiar with gcc internals, as you'll have guessed). I meant "pointer type conversion" perspective. IOW, this particular particular ip can never point to a struct b, but not because the pointers' static types are incompatible. > use ((struct b *)ip)->j then ip should point to an object with effective type > of struct b. > But, I'm afraid we can't warn about this for -Wstrict-aliasing=3, because that > would lead to too many false positives. I see three distinct cases: #1 - 'ip' has dynamic type 'int *' (can also be some "struct *" that has int as first field) extern int *ip; ((struct b *)ip)->j; #2 - 'ip' has dynamic type "final" 'int *' static struct b b; struct a *ap=(struct a *)&b; int *ip = &ap->k; ((struct b *)ip)->j; #3 - 'ip' has dynamic type "final" 'struct b *'. static struct b b; struct a *ap=(struct a *)&b; int *ip = &ap->i; ((struct b *)ip)->j; Sounds like gcc doesn't distinguish #1 from #2. IOW, it's missing some kind of tracking whether the dynamic type is "final"?
