Yes, no decimal point + no exponent would work without overhead to detect integers, but that wouldn't solve the proposed automatic long->double overflow, which is what I meant. My current idea is to default to double and optionally support any of long, BigInt and "Decimal" (BigInt+exponent), where integer overflow only works for long->BigInt.
Ah I see.
I have to say, if you are going to treat integers and floating point numbers differently, then you should store them differently. long should be used to store integers, double for floating point numbers. 64 bit signed integer (long) is a totally reasonable limitation for integers, but even that would lose precision stored as a double as you are proposing (if I'm understanding right). I don't think BigInt needs to be brought into this at all really.
In the case of integers met in the parser which are too large/small to fit in long, give an error IMO. Such integers should be (and are by other libs IIRC) serialised in the form "1.234e-123" to force double parsing, perhaps losing precision at that stage rather than invisibly inside the library. Size of JSON numbers is implementation defined and the whole thing shouldn't be degraded in both performance and usability to cover JSON serialisers who go beyond common native number types.
Of course, you are free to do whatever you like :)
