Deeply inferred function argument's type wrongly cannot be assigned to a matching generic parameter

[Raiondesu]

TypeScript Version: 2.9.0-dev.20180505

Search Terms:
generic function parameter is not assignable
deep type wrongly cannot be assigned to generic
function generic type is not inferred correctly

Code

Let's say we have a basic generic function that checks its arguments and does something to them according to a strategy function if both "exist":

function baseMerge<T>(parent: T, child: T, strategy: (parent: T, child: T) => T) {
  if (parent != undefined && child != undefined) {
    return strategy(parent, child)
  } else if (parent == undefined && child) {
    return child
  } else if (child == undefined && parent) {
    return parent
  } else {
    return undefined
  }
}

Then, trying to implement a strategy with a generic argument:

const pipe = <F extends (...args: any[]) => any[]>(parent: F, child: F) => baseMerge<F>(
  parent,
  child,
  (p, c) => (...args) => c(...p(...args)) // Here, the strategy just pipes the two array returning functions, one into the other
)

Expected behavior:

The code just compiles as usual.

Actual behavior:

Error on this line:

(p, c) => (...args) => c(...p(...args))

Type (...args: any[]) => any[] is not assignable to type F

Only a strict typecast fixes this:

(p, c) => ((...args) => c(...p(...args))) as F

which seems wrongly explicit to me, as the assigned type is exactly the same as the inferred one.

It can be viewed as unexpected behavior as in almost exactly the same use-case (outside of the function that wraps the generic) the issue is absent:

type f = (...args: any[]) => any[];
type Pipe<F extends f> = (p: F, c: F) => F;

// No error here, surprisingly
const a: Pipe<f> = (p: f, c: f) => (...args) => c(...p(...args))

Playground Link:

https://www.typescriptlang.org/play/#src=function%20baseMerge%3CT%3E(parent%3A%20T%2C%20child%3A%20T%2C%20strategy%3A%20(parent%3A%20T%2C%20child%3A%20T)%20%3D%3E%20T)%20%7B%0D%0A%20%20if%20(parent%20!%3D%20undefined%20%26%26%20child%20!%3D%20undefined)%20%7B%0D%0A%20%20%20%20return%20strategy(parent%2C%20child)%0D%0A%20%20%7D%20else%20if%20(parent%20%3D%3D%20undefined%20%26%26%20child)%20%7B%0D%0A%20%20%20%20return%20child%0D%0A%20%20%7D%20else%20if%20(child%20%3D%3D%20undefined%20%26%26%20parent)%20%7B%0D%0A%20%20%20%20return%20parent%0D%0A%20%20%7D%20else%20%7B%0D%0A%20%20%20%20return%20undefined%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Aconst%20pipe%20%3D%20%3CF%20extends%20(...args%3A%20any%5B%5D)%20%3D%3E%20any%5B%5D%3E(parent%3A%20F%2C%20child%3A%20F)%20%3D%3E%20baseMerge%3CF%3E(%0D%0A%20%20parent%2C%0D%0A%20%20child%2C%0D%0A%20%20(p%2C%20c)%20%3D%3E%20(...args)%20%3D%3E%20c(...p(...args))%0D%0A)%0D%0A%0D%0A%0D%0A%2F%2F%20No%20error%20here%2C%20however%0D%0Atype%20f%20%3D%20(...args%3A%20any%5B%5D)%20%3D%3E%20any%5B%5D%3B%0D%0Atype%20aPipe%3CF%20extends%20f%20%3D%20f%3E%20%3D%20(p%3A%20F%2C%20c%3A%20F)%20%3D%3E%20F%3B%0D%0A%0D%0Aconst%20a%3A%20aPipe%20%3D%20(p%3A%20f%2C%20c%3A%20f)%20%3D%3E%20(...args)%20%3D%3E%20c(...p(...args))%0D%0A

Related Issues:
#23705 seems related to me. But as it's not quite the same and the expected result is different (success instead of an error), I decided to create a new one. With a playground. ^_^

Unrelated Issues:
This issue DOES NOT seem to be related to #23763, #19388, #17071 and #13442 (as one might think at first) due to the absence of Partial<T> (inferred type and generic type are exactly the same in my case) as well as wrong assumptions about type inference.

P.S. Tried as hard as I could to search for duplicates and alternate solutions. Found none.

[mhegazy]

Generic type F is assignable to its constraint (...args: any[]) => any[], but the opposite is not true. consider calling your function pipe with { (...args: any[]) => any; prop: number; }.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.