Fix WeakSet interface

[falsandtru]

Fixes #14840

cc @Andy-MS

[ghost]

@mhegazy @rbuckton Any idea what PR fixed it so you could have a one declaration one a type parameter constraint and one without?

[falsandtru]

You mean lodash definitions?

[ghost]

I meant a pull request to the TypeScript compiler, that would allow the lib.d.ts definition to use WeakSet<T extends object> and lodash to use WeakSet<T>. Just making sure we've officially fixed this.

[falsandtru]

Sorry, I missed to find WeakSet because page loading is not complete. I'll fix lodash definitions.

[ghost]

That may cause errors for people using lodash with earlier TypeScript versions though. We have no way of changing these both at the same time unfortunately. So it would be best to support backwards-compatibility so lib.d.ts could declare a constraint but lodash wouldn't have to.

[falsandtru]

I made DefinitelyTyped/DefinitelyTyped#21273. This patch don't have unfavorite incompatibility.

[mhegazy]

I think we need a fix to allow defaults on only one declaration as @Andy-MS noted in #19756 (comment)

[falsandtru]

DefinitelyTyped/DefinitelyTyped#21273 is merged. So we can merge this PR.

[ghost]

The problem is with interface WeakSet<T> { } at the bottom of lodash/index.d.ts. If you change that to interface WeakSet<T extends object> { } right now you'll get a compile error. But if this PR went in then it would have to be written that way to stay compatible, but then it wouldn't work on old TypeScript versions.

[falsandtru]

Seems like it is impossible to fix WeakSet interface without breaking changes. We can never fix WeakSet?

[mhegazy]

We need a fix for #20018 first.

[falsandtru]

Now can you merge?