Not always. For instance, C has a lot of stuff around casting, memory management, etc. that is semantically legal but often good to warn about. Other times, it can warn about a possible stack overflow. C has tons of behavior which is not technically defined but has been agreed upon by most compiler implementers (and doesn't warn), though admittedly, there are also cases where you'll get different results.
That's just what I meant. Can't change C, so issue a warning. For example,
for (i = 0; i < 10; ++i);
sum += i;
if (a < b < c)
The fact that these pointless and buggy constructs persist in C is evidence that the language cannot be changed, hence a warning is used.
if (x);
In the olden days (1989) people were more willing to accept change in C, such as value-preserving rather than sign-preserving integer promotion rules, and some improvements to the C preprocessor.
if (x) foo(); // accepted
if (x) ; // rejected
if (x) { } // accepted