#include <optional>
int f(bool c) {
    std::optional<int> x;
    if (c) x = 3;
    return *x;
}

When I compile this with -fsanitize=undefined, memory I don't get any code generated to confirm that x did get initialised; but unlike an unadorned int x; version, I also don't get a compile time warning about the possible oversight. So undefined behaviour slips right through.

In this case we get something consistent with erroneous behaviour, where the result is at least predictable even if it's not right. However the intent of using the sanitiser has been undermined by premature optimisation.

The following version appears to behave as expected:

#include <optional>
int f(std::optional<int> x) {
    return *x;
}

Where the generated code does indeed check that there's a valid result available before returning it.