Awaited<T> not resolved to T when T is a generic type

[frank-weindel]

Bug Report

I thought this issue might be related to #46543, but that issue seems to apply to ALL ts versions. This issue is a regression between 4.4.4 and 4.5. In addition to reporting this bug I offer a very simple workaround.

🔎 Search Terms

generic, index, indexed record, type resolution, promise, awaited, workaround

🕗 Version & Regression Information

• This changed between versions 4.4.4 and 4.5
• Edit (3/7/22): Still an issue in 4.5.5 and 4.6.2

Bug started in 4.5 release.

Not fixed in the nightly version as of December 14th 7am ET.

⏯ Playground Link

The example below is purposely contrived in order to express the issue in simple terms.

Playground link with relevant code

💻 Code

export type GenericStructure<
  AcceptableKeyType extends string = string
> = Record<AcceptableKeyType, number>;

/**
 * structure[key] assignment compiles prior to 4.5, but fails in 4.5.3
 */
async function brokenExample<AcceptableKeyType extends string = string>(structurePromise: Promise<GenericStructure<AcceptableKeyType>>, key: AcceptableKeyType): Promise<void> {
  const structure = await structurePromise;
  structure[key] = 1;
}

/**
 * Possible workaround by explicit typing of 'structure'
 */
async function workAroundExample<AcceptableKeyType extends string = string>(structurePromise: Promise<GenericStructure<AcceptableKeyType>>, key: AcceptableKeyType): Promise<void> {
  const structure: GenericStructure<AcceptableKeyType> = await structurePromise;
  structure[key] = 1;
}

🙁 Actual behavior

In brokenExample():

The type of const structure is implicitly resolved to Awaited<GenericStructure<AcceptableKeyType>>

Assignment of structure[key] = 1; results in this error:

Type 'number' is not assignable to type 'Awaited<GenericStructure<AcceptableKeyType>>[AcceptableKeyType]'.(2322)

🙂 Expected behavior

In brokenExample():

The type of const structure should be implicitly resolved to GenericStructure<AcceptableKeyType>

Assignment of structure[key] = 1; should not result in an error.

[NtsDK]

Got the same problem. Workaround works, thank you)

[connor4312]

I believe I ran into a similar issue in VS Code. On 4.6.0 nightly, this is broken:

async function filterDefined<T>(arr: Promise<T | undefined>[]): Promise<T[]> {
    const r = await Promise.all(arr);
    return r.filter((r): r is T => r !== undefined);
}

But adding a type annotation makes it work:

async function filterDefined<T>(arr: Promise<T | undefined>[]): Promise<T[]> {
    const r: (T | undefined)[] = await Promise.all(arr);
    return r.filter((r): r is T => r !== undefined);
}

[jctaoo]

I also ran into a similar issue:

class Test<Model> {
  constructor(private model: Model) {
  }
  
  async getObj(): Promise<Partial<Model>> { 
    return this.model;
  }
  
  async getKey<Key extends keyof Model>(obj: Model, key: Key): Promise<Model[keyof Model] | undefined> {
    const model = await this.getObj();
    // error: Type 'Awaited<Partial<Model>>[Key]' is not assignable to type 'Model[keyof Model] | undefined'.
    return model[key];
  }
}

But when I removed Partial, it will works will.

Typescript Playground

[frank-weindel]

Verified that this is still an issue in 4.6.2.

Playground

[frank-weindel]

Verified that this is still an issue in 4.8.0-beta

Playground

[frank-weindel]

Thanks @rbuckton!