Incorrect type inferred in Array reduce function

[jimador]

TypeScript Version: 3.9.2/3.8.3/Nightly

Search Terms:

• array reduce
• reduce type inference

Code

type SomeParamType = { hello: string; world: string; }

function brokenInference<T>(someArg: SomeParamType): boolean {
  type KeyType = keyof typeof someArg
  const keys = (Object.keys(someArg) as KeyType[])

  return keys.reduce((previousValue, currentValue) => {
      const result = currentValue == 'hello world'
      return previousValue && result
    }, true)
}

function correctInference(someArg: SomeParamType): boolean {
  type KeyType = keyof typeof someArg
  const keys = (Object.keys(someArg) as KeyType[])
  
  return keys.reduce<boolean>((previousValue, currentValue) => {
      const test = someArg[currentValue] === 'hello world'
      return previousValue && test
    }, true)
}

function correctInference1(someArg: SomeParamType): boolean {
  type KeyType = keyof typeof someArg
  const keys = (Object.keys(someArg) as KeyType[])
  const value = keys.reduce((previousValue, currentValue) => {
      const test = someArg[currentValue] === 'hello world'
      return previousValue && test
    }, true)
  return value
}

Expected behavior:
The type of keys.reduce(...) should be inferred as boolean

Actual behavior:
The type of keys.reduce(...) is inferred as keyof T.

Notes
From what I can tell, there are currently 3 definitions for Array.reduce.

reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T): T;
reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T, initialValue: T): T;
reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: readonly T[]) => U, initialValue: U): U;

When the reduce function is called from const result = (item: T) => keys.reduce TS appears to be inferring the signature of reduce to be reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: readonly T[]) => U, initialValue: U): U;. When the function is inlined, the signature of reduce is inferred to be reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T, initialValue: T): T;.

Question:

Is this signature needed?

reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T, initialValue: T): T

This signature would seem to satisfy those constraints.

reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: readonly T[]) => U, initialValue: U): U

Playground Link:
3.9.2
3.8.3
Nightly

Related Issues:
This issue appears to be similar to #33886, #25454, #33886, #29604

[RyanCavanaugh]

Please add to #36554. A shorter repro would be ideal, if possible. Thanks!

[jimador]

Please add to #36554. A shorter repro would be ideal, if possible. Thanks!

Added comment to #36554 and I made the repro shorter.

[RyanCavanaugh]

Thank you! We'll track progress over there as we collect more use cases.