how generic type and type inference works changed

[gogoyqj]

TypeScript Version: 3.6.3+

Search Terms:

Code
(https://github.com/gogoyqj/tkit/blob/master/packages/model/__tests__/createModelSamples/testPutCallInModelOK.ts)

createModel
[model example]

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
export interface Reducers<M> {
 [doSomething: string]:
 | (<P extends never>(state: Readonly<M>, action: P) => M)
 | (<P extends AbstractAction>(state: Readonly<M>, action: P) => M);
}
export default function createModel<M, R extends Reducers<M>, E extends Effects>(model: {
 namespace: string;
 state: M;
 reducers: R;
 effects?: E;
}) {
...
}

const model = createModel({
 namespace: 'model',
 state: modelWithEffectsState,
 reducers: {
 doSomething: (state, action: { id: number }) => ({ ...state })
 },
 effects: {
 }
});

const { actions } = model;
actions.doSomething({ id: 1 });

Expected behavior:

from 3.3.+ to 3.5.,

while invoking createModel, the pass-in object's properties state's type determines reducers['doSomething'] first argument's type and its type is the type of modelWithEffectsState,

and reducers['doSomething'] second argument's type is the type of model.actions.doSomething's argument type - really wonderful features by generic and type inference

Actual behavior:

while since 3.6.3 reducers['doSomething'] first argument's type becomes any,

is there any way to recover these features, or changes i can make to make createModel works again?

help needed

Playground Link:

Related Issues:

[gogoyqj]

and in simpler scenario it works ok

function caller2<
 M,
 P extends {
 [doSomething: string]: <P extends any>(state: Readonly<M>, action: P) => M;
 }
>(o: { state: M; reducers: P }) {
 return o;
}

caller2({
 state: { name: '2' },
 reducers: {
 '2': state => ({ ...state, name: '' })
 }
});

and little complex fail

function caller2<
 M,
 P extends {
 [doSomething: string]: 
 | (<P extends never>(state: Readonly<M>, action: P) => M)
 | (<P extends{}>(state: Readonly<M>, action: P) => M);
 }
>(o: { state: M; reducers: P }) {
 return o;
}

caller2({
 state: { name: '2' },
 reducers: {
 '2': state => ({ ...state, name: '' })
 }
});

[sandersn]

skipper (@gogoyqj) This might be fixed in 3.6.3. Can you try typescript@next?

[gogoyqj]

Nathan Shively-Sanders (@sandersn)，thanks for ur suggestion
i've tried "typescript": "^3.7.0-dev.20190907", but the problem still exists.

and i change the generic R's definition

from:

export interface Reducers<M> {
 [doSomething: string]:
 | (<P extends never>(state: Readonly<M>, action: P) => M)
 | (<P extends AbstractAction>(state: Readonly<M>, action: P) => M);
}

to:

export interface Reducers<M> {
 [doSomething: string]:
 (<P extends AbstractAction>(state: Readonly<M>, action: P) => M);
}

then it works - so does typescript version 3.6.2

[weswigham]

Yeah, I can confirm that for

type Effects = {};
type AbstractAction = {};


export interface Reducers<M> {
 [doSomething: string]:
 | (<P extends never>(state: Readonly<M>, action: P) => M)
 | (<P extends AbstractAction>(state: Readonly<M>, action: P) => M);
}
export default function createModel<M, R extends Reducers<M>, E extends Effects>(model: {
 namespace: string;
 state: M;
 reducers: R;
 effects?: E;
}) {

}

const modelWithEffectsState = {};

const model = createModel({
 namespace: 'model',
 state: modelWithEffectsState,
 reducers: {
 doSomething: (state, action: { id: number }) => ({ ...state })
 },
 effects: {
 }
});

in 3.5, state is inferred as Readonly<{}>, while now it's any.

[weswigham]

Our changes to compareSignaturesIdentical cause the signatures to no longer merge in contextual typing (after all, they have distinct, unique type parameters), which then cause state to not have a contextual type applied. This is maybe a little bit too strict for contextual typing, since the type parameters are going to (sometimes, like in this case) become instantiated anyway.

[weswigham]

Regressed by #32049, needs a fix like #31029, but to compareSignaturesIdentical, rather than signatureRelatedTo.

[weswigham]

Hmmm, so sadly I think I can only say that this really only worked essentially because of the bug fixed in #32049 - the two signatures really aren't identical - even if we try to do better and produce inferences for P before checking, since one of the constraints is never, the annotation isn't assignable to that one in that case, P becomes never in that signature, and we see that (state: Readonly<{}>, action: never) => {} isn't the same as (state: Readonly<{}>, action: { id: number }) => {}, and today we only make a contextual signature if the component signatures are identical, so... eck. I imagine that's why one of the signatures is has a P extends never rather than just a never, to create the erroneous "identical" behavior, which no longer occurs.

[gogoyqj]

seems in early version[ 3.3?]

doSomething: state => ({ ...state })

is not compatible with:

doSomething: (<P extends AbstractAction>(state: Readonly<M>, action: P) => M)

but compatible with:

doSomething: (<P extends never>(state: Readonly<M>, action: P) => M)

that's why i using union

| (<P extends never>(state: Readonly<M>, action: P) => M)
| (<P extends AbstractAction>(state: Readonly<M>, action: P) => M)

and since version 3.4, 3.5, 3.6 are both compatible - but 3.6 works little differently

honestly maybe it's a bravo improvement not a bug since version 3.4 and i can simplify my code