Union type conflicts with overloaded function

[donaldpipowitch]

TypeScript Version: 2.4.1

Code

type Foo = { bar: string };

declare function foos(f: Foo);
declare function foos(f: Foo[]);

function foos2(f: Foo | Foo[]) {
  foos(f);
  // Argument of type 'Foo | Foo[]' is not assignable to parameter of type 'Foo[]'.
  //  Type 'Foo' is not assignable to type 'Foo[]'.
  //    Property 'length' is missing in type 'Foo'.
}

Expected behavior:

foos accepts Foo and Foo[]. foos2 accepts Foo | Foo[]. I think foos2 can just pass its argument to foos without problems.

Actual behavior:

Argument of type 'Foo | Foo[]' is not assignable to parameter of type 'Foo[]'.
  Type 'Foo' is not assignable to type 'Foo[]'.
    Property 'length' is missing in type 'Foo'.

[jcalz]

I've wondered about unions/intersections of parameters to overloaded/generic functions before. My guess is that it's a good thing to want but might be too expensive for the type checker to do this. Can anyone link to a canonical issue for this type of thing?

Of course in your case you can work around this in any number of ways. The best way is probably to declare the function to take Foo | Foo[] in the first place instead of overloading it.

If you are trying to fix without redeclaring the function, you can cast it:

function foos2(f: Foo | Foo[]) {
  (foos as (f: Foo | Foo[]) => any)(f); // success
}

And in general, you can force TypeScript to notice that an overloaded or generic function can take a union:

var intersectFunction:
  <A1, R1, A2, R2> (func: ((a: A1) => R1) & ((a: A2) => R2)) => ((a: A1 | A2) => R1 | R2)
  = (func => func);

function foos2(f: Foo | Foo[]) {
  intersectFunction<Foo,any,Foo[],any>(foos)(f); // success
};

I agree it would be nice if TypeScript just noticed without being forced to. It doesn't, because _________________________ (someone fill in the blank pls, thx!)

[RyanCavanaugh]

Tracked at #14107 and #1805.

What search terms did you use? I'd like to edit their titles so people can find them more easily

[jcalz]

If I search for "generic", "overload", "union", and "intersection", issue #14107 is in the top several results, so it was probably just myopia on my part. Thanks!

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.