Incorrect type inference of Promise.all when spreading Promise arrays

[peterjuras]

TypeScript Version: 4.0.2 (but also the version in the Playground)

Search Terms:

Promise.all type inference

Code

async function test() {

    const promiseNumber = Promise.resolve(1);
    const promiseVoid = async () => {}

    await Promise.all([
        promiseNumber,
        ...[promiseVoid()]
    ])
}

Expected behavior:

The Promise.all call should not throw any compile time errors due to type mismatches, since Promise.all simply returns the values of the array and waits until resolution if the value is a promise.

Actual behavior:

A compile time error is thrown:

No overload matches this call.
  The last overload gave the following error.
    Argument of type '(Promise<number> | Promise<void>)[]' is not assignable to parameter of type 'Iterable<number | PromiseLike<number>>'.
      The types returned by '[Symbol.iterator]().next(...)' are incompatible between these types.
        Type 'IteratorResult<Promise<number> | Promise<void>, any>' is not assignable to type 'IteratorResult<number | PromiseLike<number>, any>'.
          Type 'IteratorYieldResult<Promise<number> | Promise<void>>' is not assignable to type 'IteratorResult<number | PromiseLike<number>, any>'.
            Type 'IteratorYieldResult<Promise<number> | Promise<void>>' is not assignable to type 'IteratorYieldResult<number | PromiseLike<number>>'.
              Type 'Promise<number> | Promise<void>' is not assignable to type 'number | PromiseLike<number>'.
                Type 'Promise<void>' is not assignable to type 'number | PromiseLike<number>'.
                  Type 'Promise<void>' is not assignable to type 'PromiseLike<number>'.
                    Types of property 'then' are incompatible.
                      Type '<TResult1 = void, TResult2 = never>(onfulfilled?: ((value: void) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined) => Promise<...>' is not assignable to type '<TResult1 = number, TResult2 = never>(onfulfilled?: ((value: number) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<...>) | null | undefined) => PromiseLike<...>'.
                        Types of parameters 'onfulfilled' and 'onfulfilled' are incompatible.
                          Types of parameters 'value' and 'value' are incompatible.
                            Type 'void' is not assignable to type 'number'.(2769)

Workaround:

Setting the generic type argument of Promise.all to void | number works, but this is a step backwards after TypeScript added better Promise.all inference and it should know that the first argument that will be returned from the call will be a number:

// Workaround which throws no errors:
async function test() {

    const promiseNumber = Promise.resolve(1);
    const promiseVoid = async () => {}

    await Promise.all<number | void>([
        promiseNumber,
        ...[promiseVoid()]
    ])
}

Playground Link:

https://www.typescriptlang.org/play?#code/IYZwngdgxgBAZgV2gFwJYHsI2QUxMgCgEoYBvAKHJmpik3xgAcAndAW1RBwDkE2AjHMxgBeGAAVWHLgDpmedABsAbjgIBGIgG4qNOhAYt2nHADV0qACaiYoSLGKiAfGQC+lGrYDuwVMglSJjLAiooEANq6ntRG0jx8gswANFHRMunhsSbmVsQAuql5ROSuQA

Related Issues:

• Incorrect type inference in promise.all #35626

[timsuchanek]

I have this issue as well.
It came up in an issue in Prisma Client

It happens in the following code:

declare function a(): Promise<number>
declare function b(): Promise<string>

async function main() {
  const x = await Promise.all(
    [
      ...[2].map(() => a()),
      b()
    ]
  )
}

Playground Link

Could variadic tuples maybe tackle this problem?

[bfricka]

I opened this same issue: #43157
But closed it 10 minutes later when I double-checked the playground and realized it wasn't specific to 4.2.

[cbdeveloper]

Is my error related to this issue?

"typescript": "^4.2.4"

Note: All those get...() functions return Promise<SomeType>.

With a single promise, it's all fine. But as soon as I pass more than one promise to the array, it shows the following error:

Full error content:

No overload matches this call.
  The last overload gave the following error.
    Argument of type '(Promise<AdminFlags> | Promise<{ all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }>)[]' is not assignable to parameter of type 'Iterable<AdminFlags | PromiseLike<AdminFlags>>'.
      The types returned by '[Symbol.iterator]().next(...)' are incompatible between these types.
        Type 'IteratorResult<Promise<AdminFlags> | Promise<{ all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }>, any>' is not assignable to type 'IteratorResult<AdminFlags | PromiseLike<AdminFlags>, any>'.
          Type 'IteratorYieldResult<Promise<AdminFlags> | Promise<{ all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }>>' is not assignable to type 'IteratorResult<AdminFlags | PromiseLike<AdminFlags>, any>'.
            Type 'IteratorYieldResult<Promise<AdminFlags> | Promise<{ all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }>>' is not assignable to type 'IteratorYieldResult<AdminFlags | PromiseLike<AdminFlags>>'.
              Type 'Promise<AdminFlags> | Promise<{ all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }>' is not assignable to type 'AdminFlags | PromiseLike<AdminFlags>'.
                Type 'Promise<{ all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }>' is not assignable to type 'AdminFlags | PromiseLike<AdminFlags>'.
                  Type 'Promise<{ all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }>' is not assignable to type 'PromiseLike<AdminFlags>'.
                    Types of property 'then' are incompatible.
                      Type '<TResult1 = { all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }, TResult2 = never>(onfulfilled?: ((value: { all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }) => TResult1 | PromiseLike<...>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLik...' is not assignable to type '<TResult1 = AdminFlags, TResult2 = never>(onfulfilled?: ((value: AdminFlags) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<...>) | null | undefined) => PromiseLike<...>'.
                        Types of parameters 'onfulfilled' and 'onfulfilled' are incompatible.
                          Types of parameters 'value' and 'value' are incompatible.
                            Type '{ all: BrandItem[]; allValues: string[]; byValue: Record<string, BrandItem>; }' is missing the following properties from type 'AdminFlags': showPrices, showAdsts(2769)
lib.es2015.iterable.d.ts(226, 5): The last overload is declared here.

What could this be?