Comparison of `num::rational::Ratio` is incorrect with overflow

[huonw]

extern crate num;
use num::rational::Ratio;

fn main() {
    let big = Ratio::new(128u8, 1);
    let small = big.recip();
    assert!(big > small);
}

fails the assertion. (It's comparing 128 with 1/128.)

This, or something similar, happens for any type that can overflow (I guess one could argue that fixed size types are "unsupported" for rational numbers, and so this overflow leads to unspecified behaviour).

[huonw]

It appears Haskell's Data.Ratio has the same behaviour wrt overflow/wrap-around.

[lilyball]

Assuming this can't be fixed in any sane way, one option is to trigger task failure on overflow during comparison (e.g. by using .checked_mul() and failing if it returns None, although this would add the CheckedMul bounds).

[huonw]

No change. (Updated the code example)

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#333

[rust-highfive]

This issue has been moved to the RFCs repo: rust-num/num#7