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 (isOneOf<TokenType::OpeningSquareBracket, TokenType::OpeningRoundBracket, TokenType::Comma, TokenType::Minus>(last_token))
                    return false;
            }
            else if (pos->type == TokenType::Number)
            {
                if (!isOneOf<TokenType::OpeningSquareBracket, TokenType::OpeningRoundBracket, TokenType::Comma, TokenType::Minus>(last_token))
                    return false;
            }
            else if (isOneOf<TokenType::StringLiteral, TokenType::Minus>(pos->type))
            {
                if (!isOneOf<TokenType::OpeningSquareBracket, TokenType::OpeningRoundBracket, TokenType::Comma>(last_token))
                    return false;
            }
            else
            {
                break;
            }
            
            
            data_end = pos->end;
            last_token = pos->type;
            ++pos;
        }
        if (!stack.empty())
            return false;
    }
    else
        return false;
    ASTPtr type_ast;
    if (ParserToken(TokenType::DoubleColon).ignore(pos, expected)