Type inference for union type passed to overloaded generic function seems broken

[nickjs]

TypeScript Version: 2.7.0-dev.20171121, 2.6.1

Type unions coming specifically from an interface and passed to an overloaded generic constructor (like new Date) seems to break the type inference. Neither of the possible constructor overloads are matched, even though both parts of the type union are supported by the constructor.

Code

interface Foo {
  publishDate: Date | string;
}

const source: Foo = { publishDate: new Date() };
const target = new Date(source.publishDate);

Expected behavior:

target => 2017-11-22T04:08:13.477Z

Actual behavior:

Argument of type 'string | Date' is not assignable to parameter of type 'VarDate'. Type 'Date' is not assignable to type 'VarDate'.

[mhegazy]

looking at the spec, seems like Date is a valid argument to Date constructor. (https://www.ecma-international.org/ecma-262/6.0/#sec-date-value).

PRs welcomed. we should also combine the existing overloads to be:

    new(value: number | string | Date): Date;

[nickjs]

Hey @mhegazy, thanks for checking into this. You are correct, Date is a valid argument, but that already works in TypeScript, that's not the behavior this issue calls attention to. Simply passing a Date works fine: new Date(new Date()) => Date. What breaks is passing a type union of two compatible types to an overloaded constructor. Using Date here is just an example, as its constructor can accept both string and Date. Passing either of these directly works great. But passing a value of type string | Date where the specific value isn't known at compile time, that's when the compiler breaks on the constructor.

Let me know where I can provide more details! Thanks again.

[mhegazy]

Unions are not distributed over calls at the moment. in other words, new Date(x) where x is string | Date, it only works iff we have an overload that accepts string | Date, which does not exist. we do have one for each. so we need to combine them into one overload as noted earlier.

[nickjs]

Unions are not distributed over calls at the moment

I think that's the heart of the issue. I'm happy to change the Date constructor to string | number | Date, but that doesn't solve the root cause and wouldn't fix the issue for everything other than Date. Is there already an issue or roadmap item for distributing unions over overloaded calls?

[mhegazy]

Please see #14107

[nickjs]

Thanks @mhegazy, that context is super helpful. I'm going to close this issue as duplicate of #14107, and open a PR specifically to change the typing on the Date constructor, since that seems to be the best course of action here.

[mhegazy]

We should combine the two Date signatures nevertheless.