Impl PartialOrd for Number

[remexre]

It seems possible to implement both of these for Number.

[dtolnay]

Integer to float comparisons are hard. In particular, (i as f64) < f is not always equal to i < (f as u64). Any ideas for how to implement this?

[clarfonthey]

If #244 were done then perhaps this could be implemented more easily. We also know that the number isn't NaN so we could implement Ord.

[remexre]

I guess only PartialOrd would work, then:

given Numbers a and b:
	if a and b are both i64
		compare as i64
	if a and b are both u64
		compare as u64
	if a is f64 and b is f64
		compare as f64

	if a is u64 and b is i64
		if b < 0
			(a>b)
		else
			compare a vs (b as u64)
	if a is i64 and b is u64
		invert compare b, a

	if a is f64 and b is u64
		if b < max-int-representable-by-f64 (2^53?)
			compare a, (b as f64)
		else if b < floor(a)
			(b<a)
		else if b > ceil(a)
			(a>b)
		else
			undecidable
	if a is f64 and b is i64
		if b < 0
			invert compare -a, -b
		else
			compare a, (b as u64)

... assume the rest are defined the same way

Are there any flaws with this?

[dtolnay]

As discussed in the PR: I am concerned that this would limit our ability to implement arbitrary precision number parsing in the future. In particular, #252 would have wanted to replace Number with struct Number { n: String } where PartialOrd becomes way more difficult to define.

Let's keep this issue open and follow up after arbitrary-precision numbers have been settled.