Pros:
- it is convenient to type and read numbers this way;
- Ada, Perl, C++, Go and Rust already support that;
- can support thousands as well as lakhs and crores;

Cons:
- standard SQL does not have this support;
- PostgreSQL treat `SELECT 1_000_000` as number one with `_000_000` alias: http://sqlfiddle.com/#!17/0a28f/400
- C++ has `1'000'000` instead of `1_000_000`;

What do you think?

{
            ++pos;
            if (pos < end && *pos == '>')
                return Token(TokenType::Arrow, token_begin, ++pos);
            if (pos < end && *pos == '-')
            {
                ++pos;
                return comment_until_end_of_line();
            }
            return Token(TokenType::Minus, token_begin, pos);
        }
        case '*':
            ++pos;
            return Token(TokenType::Asterisk, token_begin, pos);
        case '/':   
        {
            ++pos;
            if (pos < end && (*pos == '/' || *pos == '*'))
            {
                if (*pos == '/')
                {
                    ++pos;
                    return comment_until_end_of_line();
                }
                else
                {
                    ++pos;
                    
                    size_t nesting_level = 1;
                    while (pos + 2 <= end)