Flow: Major: Function type with Date argument accepts a function with number argument

[jedwards1211]

Flow verisons

Seems to happen on any version I've tried so far:

• 0.37
• 0.40
• 0.42
• 0.50
• 0.57.1
• 0.61

Code (Try flow link)

type DateCallback = (value: Date) => Promise<any>

function foo(value: number): Promise<any> {
  return Promise.resolve()
}

(foo: DateCallback);

Expected

7: (foo: DateCallback);
     ^ function. This type is incompatible with
7: (foo: DateCallback);
          ^ function type
This parameter is incompatible:
1: type DateCallback = (value: Date) => Promise<any>
                               ^ Date. This type is incompatible with
3: function foo(value: number): Promise<any> {
                       ^ number

Actual

No errors!

[jcready]

Reduced case:

declare var x: Date;
(x: number); // no error

[jedwards1211]

whoa!

[jedwards1211]

It must have to do with making Flow support comparisons between numbers and Dates

[ilias-t]

This a major was a major "WAT" moment for me? Is this intentional? One would expect this to throw an error:

const time: number = new Date();

[NilSet]

@jedwards1211 looks like you are correct. Removing instances of Date from the numeric test at https://github.com/facebook/flow/blob/master/src/typing/flow_js.ml#L7312-L7313 fixes this particular issue, but makes it impossible to perform numeric operations on Dates

[NilSet]

One solution might be to remove the numeric attribute of Date and force users to do Number(myDate) > Number(theirDate).

[hng0]

Agreed. Date and number share some common arithmetic operations but they're definitely not the same type.

[jedwards1211]

I think it's possible to fix this issue without breaking support for number + Date and so forth. Explicit casting doesn't have to be necessary.

[NilSet]

Funny you should mention number + Date! Turns out + stringifies Date, and all the other numeric operators numberifies Date.

Meanwhile flow happily accepts const a: number = new Date() + 12;