Stacktrace from the actual crash we saw (in this case ASAN caught a stack overflow and the stack trace repeats itself endlessly, but it will also just segfault without sanitizers), but a minimal example that should cause the same behavior is this:

struct Example {
    std::optional<uint64_t> field;
}

template<typename Inspector>
bool inspect(Inspector& ins, Example& msg) {
    return ins.object(msg).fields(ins.field("field", msg.field).fallback(std::nullopt));
}