Pick and conditional update in React.setState

[kimamula]

TypeScript Version: 2.1.4 / nightly (2.2.0-dev.20170108 )

Code

Pick is supposed to work for typing React.setState (#12793).
However, there is a problem if you want to conditionally change which attributes to update.

declare class Component<P, S> {
  setState<K extends keyof S>(f: (prevState: S, props: P) => Pick<S, K>, callback?: () => any): void;
}

declare const component: Component<{}, {
  foo: number;
  bar: string;
  someCondition: boolean;
}>;

component.setState(({ someCondition }) => {
  if (someCondition) {
    return { foo: 0 };
  } else {
    return { bar: '' };
  }
});

Expected behavior:
The code compiles successfully.

Actual behavior:
component.setState line in the above code emits an error.

error TS2345: Argument of type '({someCondition}: { foo: number; bar: string; someCondition: boolean; }) => { foo: number; } | { ...' is not assignable to parameter of type '(prevState: { foo: number; bar: string; someCondition: boolean; }, props: {}) => Pick<{ foo: numb...'.
  Type '{ foo: number; } | { bar: string; }' is not assignable to type 'Pick<{ foo: number; bar: string; someCondition: boolean; }, "foo" | "bar">'.
    Type '{ foo: number; }' is not assignable to type 'Pick<{ foo: number; bar: string; someCondition: boolean; }, "foo" | "bar">'.
      Property 'bar' is missing in type '{ foo: number; }'.

You have to write like this to avoid the error.

component.setState(({ foo, bar, someCondition }) => {
  if (someCondition) {
    return { foo: 0, bar };
  } else {
    return { foo, bar: '' };
  }
});

[RyanCavanaugh]

Non-extensibly, you could write

declare class Component<P, S> {
  setState<K1 extends keyof S, K2 extends keyof S>(f: (prevState: S) => (Pick<S, K1> | Pick<S, K2>), callback?: () => any): void;
}

declare const component: Component<{}, {
  foo: number;
  bar: string;
  someCondition: boolean;
}>;

component.setState<"foo", "bar">(({ foo }) => {
    return !!true ?  { foo: 0 } : { bar: "" };
});

In general though, TS doesn't have a way to resolve this without using a unification-based inference algorithm.