no warning with returns_nonnull

clang 3.6 brought some very useful warnings when a function argument with __attribute__((nonnull)) had a NULL check:
warning: nonnull parameter 'p' will evaluate to 'true' on first encounter [-Wpointer-bool-conversion]

With gcc and earlier clang versions, the NULL check was silently optimized away.

We just realized that the same problem remains with __attribute__((returns_nonnull)): NULL checks are silently optimized away, as in the following test case.

int *nul(void) {
     return NULL;
}

__attribute__((returns_nonnull)) int *foo(void)
{
     return nul();
}

int main(void)
{
     fprintf(stderr, "%p\n", foo());
     if (foo() == NULL) {
         fprintf(stderr, "foo is null\n");
     }
     return 0;
}

Is it planned to have a warning in that case ?

Patch to fix this is in review at http://reviews.llvm.org/D15324 . It should be able to catch simple cases like the one you posted.

Thanks for catching this! :slight_smile: