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?

readQuotedStringWithSQLStyle(s, in);
        }
        catch (const Exception &)
        {
            expected.add(pos, "string literal");
            return false;
        }
        if (in.count() != pos->size())
        {
            expected.add(pos, "string literal");
            return false;
        }
    }
    else if (pos->type == TokenType::HereDoc)
    {
        std::string_view here_doc(pos->begin, pos->size());
        size_t heredoc_size = here_doc.find('$', 1) + 1;
        assert(heredoc_size != std::string_view::npos);
        s = String(pos->begin + heredoc_size, pos->size() - heredoc_size * 2);
    }
    auto literal = std::make_shared<ASTLiteral>(s);
    literal->begin = pos;
    literal->end = ++pos;
    node = literal;
    return true;
}
template <typename Collection>
struct CollectionOfLiteralsLayer
{
    explicit CollectionOfLiteralsLayer(IParser::Pos & pos) : literal_begin(pos)