Description

Chris Bouchard (chrisbouchard)

opened on Oct 14, 2018

Issue body actions

TypeScript Version: 3.2.0-dev.20181011
This code worked in 3.0.1, and is broken >=3.1.1.

Search Terms:
"is not assignable to type"

Code

export interface Entity {
    id: number | string;
}

export type IdOf<E extends Entity> = E['id'];

export interface EntityState<E extends Entity> {
    ids: IdOf<E>[];
    entities: { [key: string]: E, [key: number]: E };
}


export function getAllEntities<E extends Entity>(state: EntityState<E>): E[] {
    const { ids, entities } = state;
    return ids.map(id => entities[id]);
}

export function getEntity<E extends Entity>(id: IdOf<E>, state: EntityState<E>): E | undefined {
    const { ids, entities } = state;

    if (!ids.includes(id)) {
        return undefined;
    }

    return entities[id];
}

For completeness, I'll just mention that I intend for users to create subinterfaces of Entity and EntityState for their domain types (like @ngrx/entity), overriding id to be a more restrictive type. This is why the IdOf type is important. E.g.,

interface Task extends Entity {
    id: string;
    due: Date;
    title: string;
}

interface TaskState extends EntityState<Task> {
    // Inherits EntityState#ids as string[]
    currentTask?: string;
}

Expected behavior:
The code compiles without error.

Actual behavior:

BUILD ERROR
projects/entity/src/lib/entity-state.utils.ts(13,5): error TS2322: Type '{ [key: string]: E; [key: number]: E; }[E["id"]][]' is not assignable to type 'E[]'.
  Type '{ [key: string]: E; [key: number]: E; }[E["id"]]' is not assignable to type 'E'.
    Type 'Entity' is not assignable to type 'E'.
projects/entity/src/lib/entity-state.utils.ts(23,5): error TS2322: Type '{ [key: string]: E; [key: number]: E; }[E["id"]]' is not assignable to type 'E | undefined'.
  Type 'Entity' is not assignable to type 'E'.
    Type '{ [key: string]: E; [key: number]: E; }[E["id"]]' is not assignable to type 'E'.
      Type 'Entity' is not assignable to type 'E'.

Playground Link:
Playground Link

Related Issues:
I did not find any that seemed similar, but I'm also unsure what the actual underlying problem is. I did ask on StackOverflow, and they suggested I ask here.

Activity

1. mattmccutchen commented on Oct 14, 2018

   

   Matt McCutchen (mattmccutchen)

   on Oct 14, 2018

   Contributor

   More actions

   This was caused by #26698. The old behavior to check assignability of the indexed access type { [key: string]: E; [key: number]: E; }[E["id"]] to E was to take the base constraint of both the object type and the index type, which gave { [key: string]: E; [key: number]: E; }[string | number] = E, which worked. The new behavior takes the constraint of the object type, and if there isn't one, jumps straight to the base constraint of the whole indexed access type, so it goes to Entity and misses E.
2. chrisbouchard commented on Oct 14, 2018

   

   Chris Bouchard (chrisbouchard)

   on Oct 14, 2018

   Author

   More actions

   Is this considered a regression? Or, if this is the intended new behavior, is there a suggested workaround?
3. mattmccutchen commented on Oct 14, 2018

   

   Matt McCutchen (mattmccutchen)

   on Oct 14, 2018

   Contributor

   More actions

   I'd call it a regression, but it's not my opinion that matters.

   A workaround is to upcast id to string | number:

   export function getAllEntities<E extends Entity>(state: EntityState<E>): E[] {
       const { ids, entities } = state;
       const ids2: (string | number)[] = ids;
       return ids2.map(id => entities[id]);
   }
   
   export function getEntity<E extends Entity>(id: IdOf<E>, state: EntityState<E>): E | undefined {
       const { ids, entities } = state;
   
       if (!ids.includes(id)) {
           return undefined;
       }
   
       const id2: string | number = id;
       return entities[id2];
   }

   Reacted by Chris Bouchard, Titian Cernicova-Dragomir and NN
4. Wesley Wigham (weswigham)

   added

   BugA bug in TypeScriptA bug in TypeScript

   Domain: Indexed Access TypesThe issue relates to accessing subtypes via index accessThe issue relates to accessing subtypes via index access

   on Oct 16, 2018
5. Daniel Rosenwasser (DanielRosenwasser)

   assigned

   Anders Hejlsberg (ahejlsberg)

   on Oct 25, 2018
6. Daniel Rosenwasser (DanielRosenwasser)

   added this to the TypeScript 3.2 milestone on Oct 25, 2018
7. Daniel Rosenwasser (DanielRosenwasser)

   modified the milestones: TypeScript 3.2, TypeScript 3.2.1 on Nov 13, 2018
8. Daniel Rosenwasser (DanielRosenwasser)

   modified the milestones: TypeScript 3.2.1, TypeScript 3.3.0 on Nov 29, 2018
9. Anders Hejlsberg (ahejlsberg)

   modified the milestones: TypeScript 3.3, TypeScript 3.4.0 on Jan 16, 2019
10. Anders Hejlsberg (ahejlsberg)

    closed this as completedin #30769on Apr 12, 2019
11. Anders Hejlsberg (ahejlsberg)

    added

    FixedA PR has been merged for this issueA PR has been merged for this issue

    on Apr 12, 2019
12. microsoft

    locked as resolved and limited conversation to collaborators on Oct 21, 2025

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