Type Guards Don't Work on Parent Generic Object

[GeoMarkou]

Bug Report

🔎 Search Terms

type guard generic parent object

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about generic object type guard

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Dog { woof:string };
interface Cat { meow:string };
type Animal = Dog|Cat;

interface House <T> { animal: T; }
type DogHouse = House<Dog>;
function ProcessDogHouse (house:DogHouse) {};

const house:House<Animal> = {
    animal: undefined as any
};

if ('woof' in house.animal) {
    // We know the type to be House<Dog> but this still errors
    ProcessDogHouse(house);

    // Weirdly this doesn't error
    ProcessDogHouse({
        animal: house.animal
    });
}

🙁 Actual behavior

Generic types can't be narrowed by checking their child objects.

🙂 Expected behavior

Should be able to narrow a generic object by the types of its children.

[MartinJohns]

Duplicate. See this comment: #31755 (comment)

[MartinJohns]

Here's a recent duplicate that's still open: #42384

[GeoMarkou]

Thank you for finding the duplicate, I'll close this issue.