Two generic functions and composition

[polemius]

TypeScript Version: 3.9.2

Search Terms: two generic functions composition

Code

function id<T>(x: T): T {
  return x
}

function composition<A, B, C>(
  f: (p1: A) => B,
  g: (p2: B) => C,
): (p: A) => C {
  return (x: A) => g(f(x))
}

const a = (x: number) => x + 1
const comp = composition(a, id)
// Error
// Argument of type '<T>(x: T) => T' is not assignable to parameter of type '(p1: unknown) => number'
const comp2 = composition(id, a)

Expected behavior:

No errors shows.
The type of function comp2 should be (p: number) => number.

Actual behavior:

Type of function comp is (p: number) => number which is correct.
But type of function comp2 is (p: never) => number which is not correct.
Also TypeScript shows the error: Argument of type '<T>(x: T) => T' is not assignable to parameter of type '(p1: unknown) => number'

Playground Link: Link

[ahejlsberg]

This is effectively a duplicate of #30727. As I comment here, it's not immediately clear how we could do better given the left-to-right nature of our type inference algorithm. It's one of those examples that require full unification to make the correct inferences.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.