Property Type inference problem when picking property with index key type

[dumasss163]

TypeScript Version: 3.9.5

Search Terms:

• [K in keyof T]: T[K] extends V | undefined | null
• type T[keyof T] does not
• infer property type

Code

given the following 2 structures, they look like the same, i.e. a parent-child relationship

interface Category {
  children?: this[],
  parent?: this
}

interface Node {
  items?: this[],
  parent?: this
}

I want to access them the same way, by giving the relative property names, e.g. 'children'/'items' for child relation and 'parent' for parent relation, like this:

let category: Category = {};
// access children and parent of category
handle(category, 'children', 'parent'); //!! <- inference problem here

let node: Node = {};
// access items and parent of node
handle(node, 'items', 'parent'); //!! <- inference problem here

here is how I define the handle method with some helpers

type Nullable<T> = T | undefined | null;

type PickKey<T, V> = keyof {
  [K in keyof T]: T[K] extends Nullable<V> ? K : never
};

export function handle<
  T extends Record<PickKey<T, T[]>, Nullable<T[]>
    & Record<PickKey<T, T>, Nullable<T>>
  >
>(
  tree: T,
  childrenKey: PickKey<T, T[]>,
  parentKey: PickKey<T, T>,
) {
  let parent = tree[parentKey];
  if (parent) {
    let siblings = parent[childrenKey];
  }
  let children = tree[childrenKey];
  if (children) {
    for (let ii = 0; ii < children.length; ii++) {
      const child = children[ii];
    }
  }
}

Expected behavior:

It should compile

Actual behavior:

Argument of type 'Category' is not assignable to parameter of type 'Record<"children" | "parent", Category[] & Record<"children" | "parent", Nullable<Category>>>'.
  Types of property 'children' are incompatible.
    Type 'Category[] | undefined' is not assignable to type 'Category[] & Record<"children" | "parent", Nullable<Category>>'.
      Type 'undefined' is not assignable to type 'Category[] & Record<"children" | "parent", Nullable<Category>>'.
        Type 'undefined' is not assignable to type 'Category[]'.ts(2345)

Playground Link:

Related Issues:

it is related, but not really the same
#38646

[jack-williams]

Did you try any of the solutions in #12290?