Flow: Cannot compare generic with itself

[lyleunderwood]

Flow version: 0.98.0+

Expected behavior

const compareGeneric = <T: number>(a: T, b: T): boolean => 
  a < b;
// no errors

Actual behavior

const compareGeneric = <T: number>(a: T, b: T): boolean => 
  a < b;
//^ Cannot compare `T` [1] to `T` [2].

• Link to Try-Flow or Github repo: try

I suspect this is linked to the change in 0.98.0 referenced here, as it's the only thing I could find that sounded sort of related:

Fix an unsoundness with addition or logical operators when combined with generics (#6671, #7070).

Those issues point at this change: https://github.com/facebook/flow/commit/db8a41e4e2fbfca0fffddebb69917b1835789872

So the question I'm left with is, how _should_ I type this comparison function, if not in the way I've typed it? This really _looks_ like it should be fine. Math operators and === all work as expected in the same position.

[jcready]

Not an ideal workaround:

const compareGeneric = <T: number | Date>(a: T, b: T): boolean => a - b > 0;

[barretron]

2 ways I can think of:

const compareGeneric = <T: number | Date>(a: T, b: T): boolean => 
  +a < +b;

const compareGeneric2 = <T: number | Date>(a: T, b: T): boolean =>
          Number(a) < Number(b);

[lyleunderwood]

Thanks, I hate it.

Using Number looks like the clearest option to me.

Is there some underlying reason I should understand that explains why this behavior is correct? Or is this actually a bug?

[barretron]

Seems buggish since it's really easy to defeat, e.g. this doesn't error:

const compareGeneric = <T: number>(a: T, b: T): boolean => 
  Boolean(a) && a < b; // Works

Or even nonsensical things:

const compareGeneric = <T: number>(a: T, b: T): boolean => 
  isNaN(a) && a < b; // Works

But it also seems like generics aren't really needed here:

const compare = (a: number | Date, b: number | Date): boolean => 
  a < b;

[lyleunderwood]

thanks @mroch