Generic type is not correctly inferred from the function return type when it is a function expression type

[markostanimirovic]

Bug Report

🕗 Version & Regression Information

Tested with different TS v4 versions. It does not seem like a regression.

⏯ Playground Link

Playground link with relevant code

💻 Code

// === Reproduction ===

export type ActionReducer<State> = (state: State | undefined) => State;

export function createReducer<State>(initialState: State): ActionReducer<State> {
 return {} as any;
}

export function createFeature<State>(config: {
 reducer: ActionReducer<State>,
 selectors: (state: State) => unknown,
}) {}

createFeature({
 reducer: createReducer(''),
 // the `state` type is `unknown`
 // if we remove `state` argument the error will disappear
 selectors: (state) => ({}),
 // selectors: () => ({})
});

// === Workarounds ===

// === Workaround 1:
// Define return type as another generic

export function createReducer2<
 State,
 Reducer extends ActionReducer<State> = ActionReducer<State>
>(initialState: State): Reducer {
 return {} as any;
}

createFeature({
 reducer: createReducer2(123),
 // the `state` type is correctly inferred (`number` in this case)
 selectors: (state) => ({})
});

More workarounds are available at the playground link.

🙁 Actual behavior

A generic type is not correctly inferred from the function return type when it is a function expression type.

By the way, I noticed that the State type will be correctly inferred when the ActionReducer from the example above is not defined as a function expression type:

// `ActionReducer` is not a function type anymore
export type ActionReducer<T> = T | undefined;

export function createReducer<T>(initialState: T): T {
 return {} as any;
}

export function createFeature<State>(config: {
 reducer: ActionReducer<State>,
 selectors: (state: State) => unknown,
}) {}

createFeature({
 reducer: createReducer(''),
 // the `state` type is correctly inferred
 selectors: (state) => ({}),
});

🙂 Expected behavior

I'd expect the same behavior when ActionReducer is typed as a function expression.

[Nut41tank]

+1

[RyanCavanaugh]

See #47599

[markostanimirovic]

closing as duplicate