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?

{
    if (pos >= end)
        return Token(TokenType::EndOfStream, end, end);
    const char * const token_begin = pos;
    auto comment_until_end_of_line = [&]() mutable
    {
        pos = find_first_symbols<'\n'>(pos, end);    
        return Token(TokenType::Comment, token_begin, pos);
    };
    switch (*pos)
    {
        case ' ':
        case '\t':
        case '\n':
        case '\r':
        case '\f':
        case '\v':
        {
            ++pos;
            while (pos < end && isWhitespaceASCII(*pos))
                ++pos;
            return Token(TokenType::Whitespace, token_begin, pos);