ts4.6.1-rc regression: Property ... does not exist on type 'Partial<MyStorage>[K]'

[HolgerJeromin]

Bug Report

🔎 Search Terms

partial, generics

🕗 Version & Regression Information

Tested with nuget package 4.6.0 and 4.6.1-rc and "nightly" on playground with is labeled v4.6.0-dev.20220116

• This changed between versions 4.5.4 and 4.6.0

⏯ Playground Link

function __getStoredValue<K extends keyof Grid.Storage>(
    key: K
): Grid.Storage[K]['anonymousUser'] | undefined {
    let user = 'admin';

    let storageForKey = {
        anonymousUser: [1],
        users: {
            admin: [2]
        }
    } as Partial<Grid.Storage>[K];

    if (!storageForKey) {
        return undefined;
    }

    if (!user) {
        return storageForKey.anonymousUser;  // <--
    } else {
        if (storageForKey.users) {  // <--
            return storageForKey.users[user]; // <--
        }
        return undefined;
    }
}
namespace Grid {
    export interface Storage {
        columnWidths: StorageItem<number[]>;
    }
    export interface StorageItem<V> {
        /** value for anon */
        anonymousUser: V;
        /** value for each user */
        users: Record<string, V>;
    }
}
const result = __getStoredValue('columnWidths');
console.log(result);

Workbench Repro

🙁 Actual behavior

TS4.5.4 is happy to compile my code, but ts4.6.x complains on access anonymousUser or users:

Property 'anonymousUser' does not exist on type 'Partial[K]'.
Property 'users' does not exist on type 'Partial[K]'.
Property 'users' does not exist on type 'Partial[K]'.

🙂 Expected behavior

Should still compile as the property exists on the type.

[RyanCavanaugh]

Cleaned up:

interface MyObj {
    someKey: {
      name: string;
    }

    someOtherKey: {
      name: number;
    }
}

const ref: MyObj = {
  someKey: { name: "" },
  someOtherKey: { name: 42 }
}

function func<K extends keyof MyObj>(k: K): MyObj[K]['name'] | undefined {
    const myObj: Partial<MyObj>[K] = ref[k];
    
    if (myObj) {
      return myObj.name;
    }

    const myObj2: Partial<MyObj>[keyof MyObj] = ref[k];
    if (myObj2) {
      return myObj2.name;
    }
    return undefined;
}

This appears to be caused by #47109, specifically that we no longer immediately resolve Partial<MyObj>[K] to a non-generic type.

I believe this can be worked around with some minor modification (shown the repro as myObj2) which wouldn't cause any loss of correctness in working code, but we should document it in the blog regardless.

[RyanCavanaugh]

@ahejlsberg ping to confirm this is an intended break

[HolgerJeromin]

This duplicates the type information from __storage object into the function variable.

const __storage: Partial<Grid.Storage> = {...}
// inside function
const storageForKey:Partial<Grid.Storage>[keyof Grid.Storage]  = __storage[key];
// does not work:
const storageForKey:Partial<Grid.Storage>[K]  = __storage[key];

But fine enough for me. Thanks for your work.

Complete example

namespace Grid {
    export interface Storage {
        columnWidths: StorageItem<number[]>;
    }
    export interface StorageItem<V> {
        /** value for anon */
        anonymousUser: V;
        /** value for each user */
        users: Record<string, V>;
    }
}
const __storage: Partial<Grid.Storage> = {
    columnWidths: {
        anonymousUser: [1],
        users: {
            admin: [2]
        }
    }
};

function __getStoredValue<K extends keyof Grid.Storage>(
    key: K
): Grid.Storage[K]['anonymousUser'] | undefined {
    let user = 'admin';

    const storageForKey = __storage[key];
    //const storageForKey:Partial<Grid.Storage>[K]  = __storage[key]; // does not work
    //const storageForKey:Partial<Grid.Storage>[keyof Grid.Storage]  = __storage[key]; // works

    if (!storageForKey) {
        return undefined;
    }

    if (!user) {
        return storageForKey.anonymousUser;  // <--
    } else {
        if (storageForKey.users) {  // <--
            return storageForKey.users[user]; // <--
        }
        return undefined;
    }
}


const result = __getStoredValue('columnWidths');
console.log(result);

[ahejlsberg]

This is not an intended break. This change in #47109 was part of an early evolution of the PR, but should have been removed. It's an easy fix, I'll put up a PR.

[HolgerJeromin]

Successfully tested with TS 4.7.0-dev.20220220. Thanks for your quick fix.