Conflicting definitions for Set constructor causes unexpected default generic in TS 3.5

TypeScript Version: 3.4.5 vs versions after 3.5

Search Terms: set constructor setconstructor unknown

Code

let s = new Set();

Expected behavior:

In TS 3.4: we expect the default chosen param to give us a Set<{}>. In TS 3.5: we expect a Set<unknown>. That change was an intended change in TS 3.5.

Or, if Set has a default generic arg, I expect the same default chosen in 3.4 and 3.5.

Actual behavior:

In TS 3.4, this actually was a Set<any>!

It appears there are multiple overloads of the Set constructor.

lib.es2015.collection.d.ts says:

    new <T = any>(values?: ReadonlyArray<T> | null): Set<T>;

while lib.es2105.iterable.d.ts says:

    new <T>(iterable?: Iterable<T> | null): Set<T>;

Note that one has T = any and the other doesn’t.

So somehow TS 3.4 vs TS 3.5 decided to change whether to obey the any default, I think?

[evmar]

Circling back on this – I think maybe I failed to communicate the bug here.

In practice, TS3.4 => TS3.5 changed what new Set() gives you, from Set<any> to Set<unknown>. (That is too late to undo, though massively annoying for us to upgrade through.)

But the actual bug here is that I think you didn’t change the definition of Set! It still says any in the typings, something else must be going wrong.

[evmar]

Just to restate, what I am concerned about here is whether “new Set()” is supposed to give you a Set<any> or a Set<unknown>, given the conflicting definitions in the typings.

I believe the intention was for 3.5 change it to <unknown> and for that to break apps – that is at least what 3.5 did in practice – but I think the resolution of this bug is to make the typings express that intent more clearly.