| Issue |
76264
|
| Summary |
diagnose_if runs within false branches of C++ `if constexpr`, leading to false positives
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
q66
|
minimal reproducer:
```
#include <cstdio>
#include <cstring>
#include <cstddef>
void *copy(void *d, void *s, size_t n) __attribute__((diagnose_if(__builtin_object_size(d, 0) < n, "n larger than d", "error"))) {
return memcpy(d, s, n);
}
int test(long long v) noexcept
{
int r = 0;
if constexpr (sizeof(r) == sizeof(v)) {
copy(&r, &v, sizeof(v));
}
return r;
}
int main() {
return test(16);
}
```
compiling reveals:
```
$ c++ test.cc -o test
test.cc:13:31: error: n larger than d
13 | copy(&r, &v, sizeof(v));
| ^
test.cc:5:55: note: from 'diagnose_if' attribute on 'copy':
5 | void *copy(void *d, void *s, size_t n) __attribute__((diagnose_if(__builtin_object_size(d, 0) < n, "n larger than d", "error"))) {
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```
The condition `sizeof(r) == sizeof(v)` is always false in this case, yet the diagnostic is still invoked, like if it was true. This does leads to a false positive at compile-time. The workaround is obviously easy (to use `sizeof(r)` instead) but should not be necessary.
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