Infer interdependencies in object literals

[gffuma]

TypeScript Version: 4.0.5

Search Terms:

infer, object literals, interdependencies

Code

type Selector<S> = (state: S) => any

export interface Reducer<T = any> {
 (state: T | undefined, action: any): T
}

type UnboxState<R extends Reducer> = R extends Reducer<infer U> ? U : never

declare function example<R extends Reducer, S extends Selector<UnboxState<R>>>(r: R, s: S) : void

example(
 (state, action) => ({
 name: 'Gio Va'
 }),
 state => 23
)

declare function example2<R extends Reducer, S extends Selector<UnboxState<R>>>(x: { r: R, s: S }) : void

example2({
 r: (state, action) => ({
 name: 'Gio Va'
 }),
 s: state => 23
})

Expected behavior:

Infer type { name: string } to state in the example2 function as it does correctly in example function.

Actual behavior:

In the example2 function the type inferred to state is any.

Playground Link:

playground

Related Issues:

[RyanCavanaugh]

See #30134 - certain parts of inference depend on left-to-right per-parameter inference; we would need unification to infer this correctly