T[K] not assignable to Partial<T[K]>

[krishan]

TypeScript Version: 2.5.2

Code

function f<T, K extends keyof T>(t: T, k: K): Partial<T[K]> {
  return t[k];
}

Expected behavior:
No error. In my understanding, X should always be assignable to Partial<X>.

Actual behavior:

TS2322: Type 'T[K]' is not assignable to type 'Partial<T[K]>'.
  Type 'T[string]' is not assignable to type 'Partial<T[K]>'.

[krishan]

If I break down the above code into two separate functions, there's no error. Therefore I think this must be a bug.

function f<T, K extends keyof T>(t: T, k: K): Partial<T[K]> {
  return g(h(t, k));
}

function h<T, K extends keyof T>(t: T, k: K): T[K] {
  return t[k];
}

function g<T>(t: T): Partial<T> {
  return t;
}

[DanielRosenwasser]

This may be related to #18156.