Add a constraint for exact types

[jcready]

I have a generic function that by design requires that its parameters may only take exact arguments, i wish i could get such guarantee from flow by declaring my function as follows:

declare class Object {
  static entries<exact T>(object: T): Array<[$Keys<T>, $Values<T>]>;
  static entries(object: any): Array<[string, mixed]>;
  static keys<exact T>(o: T): Array<$Keys<T>>;
  static keys(o: any): Array<string>;
  static values<exact T>(object: T): Array<$Values<T>>;
  static values(object: any): Array<mixed>;
}

[goodmind]

I think this is flawed idea, because exact object is more like labeled tuple, so you can't really have supertype of all exacts

Related #7773

[jcready]

What is the proposed alternative solution to properly typing Object.values() and Object.entries() so they don't result in mixed?

[goodmind]

OCaml :\

[eilvelia]

Currently the constraint exists, you can use $Exact for this:

declare function keys<O: $Exact<any>>(o: O): Array<$Keys<O>>;
declare function keys(o: mixed): Array<string>;

declare var o1: {a:1, ...}
declare var o2: {|a:1|}
const ex1 = keys({a:1})
const ex2 = keys(o2)
const inex1 = keys(o1)

const t11: 'a' = ex1[0] // ok
const t12: 'a' = ex2[0] // ok
const t13: 'a' = inex1[0] // error

try flow