Array.prototype.reduce return type incorrectly inferred as 'any' when Array has type any[]

[vp2177]

TypeScript Version: 3.6.3

Search Terms: Array reduce type inferred bad|incorrect|any

Code

const s: Array<any> = []
const x = s.reduce((a, c) => a + '*', '')

Expected behavior: x should be inferred to be of type string

Actual behavior: x is inferred to be any

However, x is is correctly inferred to be type string, if using [].reduce(... as opposed to s.reduce(....

Compare:

with

Inferring x's type even if s is polymorphic is possible and works in other languages:

Playground Link:

Related Issues:
#21061

[jcalz]

The overloads of reduce() are in a suboptimal order when the array element type is very wide like any or unknown (if the initial value is assignable to it). If the generic call signature were moved to the front, this issue would not arise:

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

const s: Array<unknown> = []
const x = s.reduce((a, c) => a + '*', ''); // string

Playground link

I'm not sure if this rises to the level of "bug"; in any case, you could always locally merge in the above declarations in your code base to resolve it (actually you only need the generic one).

[vp2177]

Thank you!
Then I think this is a duplicate of #7014.