Optional properties should be ignored when inferring K in Pick<ConcreteType, K>

[laughinghan]

TypeScript Version: 2.8.1

Search Terms: pick, partial, optional

Code

interface Foo {
    asdf: string;
    hjkl: number;
}
let partialFoo: Partial<Foo>;

type FnThatTakesPick = <K extends keyof Foo>(x: Pick<Foo, K>) => void;
let fnThatTakesPick: FnThatTakesPick;

fnThatTakesPick(partialFoo);
// ^ Argument of type 'Partial<Foo>' is not assignable to parameter of type 'Pick<Foo, "asdf" | "hjkl">'.
//     Property 'asdf' is optional in type 'Partial<Foo>' but required in type 'Pick<Foo, "asdf" | "hjkl">'.


// but fnThatTakesPick essentially takes a partial!
// In fact, it accepts exactly the same objects:

type FnThatTakesPartial = (x: Partial<Foo>) => void;
let fnThatTakesPartial: FnThatTakesPartial;

const a = {};
fnThatTakesPick(a);
fnThatTakesPartial(a);

const b = { asdf: '' };
fnThatTakesPick(b);
fnThatTakesPartial(b);

const c = { hjkl: 0 };
fnThatTakesPick(c);
fnThatTakesPartial(c);

const d = { asdf: '', hjkl: 0 };
fnThatTakesPick(d);
fnThatTakesPartial(d);


// also, note that the reverse works:
const _ = <K extends keyof Foo>(x: Pick<Foo, K>) => {
    fnThatTakesPartial(x);
};

Expected behavior:
No typechecking errors.

Actual behavior:
The marked typechecking errors.

Playground Link

Related Issues:

• Inconsistent behaviour of Pick<T, keyof T> for readonly attributes. #14295 and Pick turns optional parameters into required ones when union types are used #20722 look similar but appear to be about Pick not propagating optionality and readonly-ness
• Computed property key names should not be widened #13948 looks similar but has to do with how Pick treats computed properties

[laughinghan]

Never mind they're not the same, apparently optional properties effectively add | undefined to the type of a key:

const b = { asdf: undefined };
fnThatTakesPick(b);    // errors if you turn on strictNullChecks
fnThatTakesPartial(b); // no error

const c = { hjkl: undefined };
fnThatTakesPick(c);    // errors if you turn on strictNullChecks
fnThatTakesPartial(c); // no error

const d = { asdf: undefined, hjkl: undefined };
fnThatTakesPick(d);    // errors if you turn on strictNullChecks
fnThatTakesPartial(d); // no error

Playground link (you have to manually turn on strictNullChecks)

via #13948 (comment)