Incorrect overload resolution in 3.2.0-rc

[Jessidhia]

TypeScript Version: 3.2.0-rc

Search Terms:
Overload

Code

import { createStore } from 'redux'

const initialStore: { foo: {} } = { foo: {} }
const store = createStore(x => x, initialStore)

Expected behavior:

The second overload of createStore should be chosen, as initialStore is not a StoreEnhancer but a preloadedState.

Actual behavior:

The StoreEnhancer, a function, overload of createStore is chosen despite initialStore being absolutely, definitely not a function, causing the type of x to be incorrectly inferred as {} | undefined.

[ahejlsberg]

The error is not that we're picking the wrong overload, but rather that none of the overloads are applicable.

Here's a simplified repro:

declare function test<T>(f: (x: T | undefined) => T, enhancer: () => void): T;
declare function test<T>(f: (x: T | undefined) => T, state: T): T;

test(x => x, { a: 'hello' });  // Ok in 3.1, error in 3.2
test(x => x || { a: 'xxx' }, { a: 'hello' });  // Ok in both

The correct inference for T is { a: string } and therefore it is an error for the arrow function to return x ({ a: string } | undefined is not assignable to { a: string }).

The reason it succeeds in 3.1 is somewhat subtle: We do multiple passes on the overloaded signatures, initially making an inference of { a: string } for T. In a later pass we end up with a contra-variant inference of { a: string } and a co-variant inference of { a: string } | undefined. In 3.1 we'd then go with the less specific inference of { a: string } | undefined, but in 3.2 we now prefer the more specific and more correct { a: string }. This changed in the commit here as part of #27028.

The fact that 3.1 gets it wrong becomes obvious if you comment out the first overload (which shouldn't really matter as it is not applicable). 3.1 then behaves the same as 3.2.

[Jessidhia]

I see. This is likely then an error with the redux type definitions -- if there is an initial state, then the reducer won't be called with undefined but rather with... the initial state.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.