TypeScript Version: 2.8.1 and 2.9.0-dev.20180329

Search Terms:

I tried actually a lot of terms around mapped types, arrays, conditional types, array member dereferencing in mapped types etc..

Code

export type RecursivePartial<T> = {
  [P in keyof T]?: T[P] extends Array<any> ? {[index: number]: RecursivePartial<T[P][0]>} :
    T[P] extends object ? RecursivePartial<T[P]> : T[P];
};

var a = {o: 1, b: 2, c: [{a: 1, c: '213'}]}
function assign<T>(o: T, a: RecursivePartial<T>) { }
assign(a, {o: 2, c: {0: {a: 2, c: '213123'}}})

Expected behavior:

T[P][0] is usable and doesn't produce an error, since T[P] extends any[] and should thus be indexable on [0] (it seems this is the way to dereference an array)

Actual behavior:
I am getting type 0 cannot be used to index type T[P].

The odd part is that behaviour-wise it seems to be working ! I can't change the type of the nested c to anything other than string and I can't create random properties or what.

Am I missing something ?