Comment by MathMonkeyMan

You returned a pointer to a temporary. It's nasty but easy to spot.

Would you return `input.c_str()`? You might think twice about that, but it's the same thing. My favorite fix is to take a `std::string_view` as the parameter.

    string_view remove_leading_spaces(string_view input) {
        auto new_start = find_if(input.begin(), input.end(), [](char ch) { return ch != ' '; });
        return string_view(new_start, input.end());
    }

The result of the undefined behavior in your program is surprising, though. As it usually is.