https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101645
--- Comment #4 from Matthew Wilcox <matthew at wil dot cx> ---
On second thoughts -Wsign-conversion is useless. Consider that it warns on
this:
unsigned long f(unsigned long x, int y) {
return x + y;
}
Both gcc and clang emit a warning, which makes it useless. That code obviously
does what the programmer intended.
What would be useful is a warning which diagnoses code which behaves
differently in infinite-precision arithmetic vs the C promotion rules.
For example,
unsigned long g(unsigned char v, int s) {
return v << s;
}
By the C standard, v is promoted to int, shifted by s and then promoted to
unsigned long. This is a fertile source of bugs as it's easy to overlook that
v will not be promoted to unsigned long first.
(by the way, this is another example where clang diagnoses with -Wsign-compare
and gcc doesn't.)
Another example plucked from real life:
long long h(unsigned int x) {
return x * 4096;
}
(slightly changed to demonstrate the problem on LP64; the original was from
unsigned long to long long, and it is only a bug on ILP32)
