Unable to infer the inner type within expansion of a nested union

[pelotom]

TypeScript Version: 3.0.1, 3.0.3, 3.1.0-dev.20180831

Search Terms: infer nested union type

Code

type Maybe<A> = { tag: 'none' } | { tag: 'some'; value: A };
type MaybeMaybeNumber =
  | { tag: 'none' }
  | {
      tag: 'some';
      value: { tag: 'none' } | { tag: 'some'; value: number };
    };
type X = Maybe<Maybe<number>> extends Maybe<Maybe<infer T>> ? T : never;
type Y = MaybeMaybeNumber extends Maybe<Maybe<infer T>> ? T : never;

Expected behavior:

The types X and Y should both be number, because MaybeMaybeNumber is just an expansion of Maybe<Maybe<number>>.

Actual behavior:

X is number, but Y is {}.

Playground Link

[ahejlsberg]

This is a design limitation of our inference algorithm. Specifically, if, during inference, we are already processing another target type with the same associated symbol (such as an instantiation of the same generic type), we do not explore the nested target as it rarely yields further inferences, but definitely generates lots of work. Obviously that's not a good strategy here.

[pelotom]

I see. Any chance of the design being revisited? 🙂

[sledorze]

@ahejlsberg is there any way to force that exploration somehow? (forcing being enforced by library authors at definition site)

[safareli]

I've checked and starting from Typescript@4.1.5 this is fixed see playground link

/cc @pelotom

[vaaas]

Since this is fixed, this issue shouldn't be open, no?

[RyanCavanaugh]

Yep!