GCC’s sanitizers actually catches most of these. The only one it wont complain about is

#include <optional>

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

because nullopt is a nullopt_t which is akin to NULL, and will read as 0 if you try to do something with it, as far as I can tell…

The span missing bounds checking isn’t a huge deal, but I guess it would make sense for them to add an at method like most other containers have…

TLDR: Use g++ -Werror -Wpedantic -Wall -fsanitize=address,undefined,null when compiling. Maybe do another run with thread, leak, or other sanitizers if needed.