Set constructor does not accept Union type of Set | undefined when strictNullChecks is enabled

[tkntkn]

TypeScript Version: typescript@3.4.0-dev.20190202

Search Terms:
set union constructor strict null undefined readonly array strictNullChecks

Code

const params = undefined as Set<any> | undefined;
const ok = params ? new Set(params) : new Set(params);
const ng = new Set(params);

compile command

tsc test.ts -target ES2017 -strict

Expected behavior:
It should be compiled successfully

Actual behavior:
Line const ng = new Set(params) raise type error:

error TS2345: Argument of type 'Set<any> | undefined' is not assignable to parameter of type 'readonly any[] | null | undefined'.
  Type 'Set<any>' is missing the following properties from type 'readonly any[]': length, concat, join, slice, and 11 more.

This error occurs only when strictNullChecks is enabled

Playground Link:
http://www.typescriptlang.org/play/#src=const%20params%20%3D%20undefined%20as%20Set%3Cany%3E%20%7C%20undefined%3B%0D%0Aconst%20ok%20%3D%20params%20%3F%20new%20Set(params)%20%3A%20new%20Set(params)%3B%0D%0Aconst%20ng%20%3D%20new%20Set(params)%3B%0D%0A
Toggle strictNullChecks on/off from "Option" menu

Related Issues:
I could not find any related issues.

[azizhk]

So there are two overloads of SetContructor
https://github.com/Microsoft/TypeScript/blob/4505eea92d6509ffea479128cb7e72706ae7f090/lib/lib.es2015.collection.d.ts#L67-L70

https://github.com/Microsoft/TypeScript/blob/afa94c527cd407fb0eed815d070f66406485a03d/src/lib/es2015.iterable.d.ts#L179-L181

I think the second one should be

interface SetConstructor {
  new <T>(iterable?: Iterable<T>): Set<T>;
}

When you split them up into parts, the resolution gets split, that is why it is working.

const ok = params ? new Set(params) : new Set(params);`
                            ^Iterable         ^ReadonlyArray | undefined