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?

ASTPtr literal_node;
        if (literal_p.parse(pos, literal_node, expected))
        {
            layers.back().arr.push_back(literal_node->as<ASTLiteral &>().value);
        }
        else if (pos->type == opening_bracket)
        {
            layers.emplace_back(pos);
            pos.increaseDepth();
        }
        else
            return false;
    }
    expected.add(pos, getTokenName(closing_bracket));
    return false;
}
template bool ParserCollectionOfLiterals<Array>::parseImpl(Pos & pos, ASTPtr & node, Expected & expected);
template bool ParserCollectionOfLiterals<Tuple>::parseImpl(Pos & pos, ASTPtr & node, Expected & expected);
namespace
{
class ICollection;
using Collections = std::vector<std::unique_ptr<ICollection>>;
class ICollection
{
public:
    virtual ~ICollection() = default;
    virtual bool parse(IParser::Pos & pos, Collections & collections, ASTPtr & node, Expected & expected) = 0;
};
template <class Container, TokenType end_token>
class CommonCollection : public ICollection