| Issue |
119928
|
| Summary |
`DiagRuntimeBehavior` (and presumably the CFG in general) incorrectly determines reachability in the presence of `__builtin_is_constant_evaluated`
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
zygoloid
|
[Testcase](https://godbolt.org/z/Wbf8zYfh6):
```c++
void f(int k) {
// No UB. But warns about UB.
const int m = __builtin_is_constant_evaluated() ? 0 : *(int*)0;
}
void g(int k) {
// Has UB. But does not warn about UB.
const int m = __builtin_is_constant_evaluated() ? k : *(int*)0;
}
```
Presumably we get `f` wrong because the CFG builder evaluates `__builtin_is_constant_evaluated()` to `false` when building a CFG for the conditional _expression_. But I'm unsure why we get `g` wrong too.
Perhaps the CFG builder could track whether it's within a manifestly constant evaluated context, and pass that information into the constant evaluator. However, with the likely upcoming changes [adding reflection to C++](http://wg21.link/P2996), there'll be other ways in which constant evaluation becomes context-dependent, so this might not be a particularly good solution in the longer term.
Another possible option would be to make the CFG builder entirely skip constant expressions, and instead just use the already-known constant values from them. The C++ language rules already require UB checking within constant expressions, so most additional CFG-based checks are likely to be redundant (though not all of them).
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