Closed
Description
TypeScript Version: 2.3.4
Code
interface Test {
cat: number;
dog: number;
}
declare function doTest<T extends keyof Test>(x: Pick<Test, T>): void;
declare var cat: { cat: number };
doTest(cat);
declare var dog: { dog: number };
doTest(dog);
declare var catOrDog: { cat: number } | { dog: number };
doTest(catOrDog); // ErrorExpected behavior: Last line should not error
Actual behavior: Property 'dog' is missing in type '{ cat: number; }'.
I understand this might be working as intended insofar that, in the last invocation of doTest, it's unclear what generic T to infer for the function. If T is 'cat'|'dog', then Pick<Test, 'cat'|'dog'> would require both cat and dog, which the union type for catOrDog does not match.
On the other hand, it seems very odd for a function to accept both an argument of type { cat: number } and type { dog: number } but not type { cat: number }|{ dog: number }.
As a work-around, it is possible to type doTest with additional generics, like so:
declare function doTest<T extends keyof CatAndDog, V extends keyof CatAndDog>(
x: Pick<CatAndDog
5355
, T>|Pick<CatAndDog, V>
): void;
doTest<"cat", "dog">(catOrDog); // No erorBut this feels like a less than ideal solution.