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 literal_begin = pos;
    bool negative = false;
    if (pos->type == TokenType::Minus)
    {
        ++pos;
        negative = true;
    }
    else if (pos->type == TokenType::Plus)  
        ++pos;
    Field res;
    if (!pos.isValid())
        return false;
    /** Maximum length of number. 319 symbols is enough to write maximum double in decimal form.
      * Copy is needed to use strto* functions, which require 0-terminated string.
      */
    static constexpr size_t MAX_LENGTH_OF_NUMBER = 319;