type inference error on Union type with keyof

[sidecus]

Seeing some type inference error with simple repro code. Not sure it's a bug or not so posting to get some insights.
Kindly note this is not a critical issue, I am just curious about the behavior.

TypeScript Version: 4.0.2 (tried 3.9 as well, same)
Code

class A {
    memberA: string = 'a';
}

class B extends A {
    memberB: string = 'b';
}

class C extends A {
    memberC: string = 'c';
}

class DummyMap<T extends A> {
    constructor(private obj: T) {
        this.obj = obj;
    }
    get<K extends keyof T>(key: K): T[K] {
        return this.obj[key];
    }
}

const map1 = new DummyMap(new B());
const func = (map: DummyMap<B> | DummyMap<C>) => {
    console.log(map.get('memberA')); // compiler error, incompatible signatures
}
func(map1);

//works
const map2: DummyMap<B> | DummyMap<C> = map1;
console.log(map2.get('memberA'));    // ok

Expected behavior:
First map.get should be able to compile.

Actual behavior:

Playground Link:
Playground repro

Related Issues:

[MartinJohns]

Duplicate of #7294. Search terms used: union callable

[sidecus]

@MartinJohns thanks! Sorry I should have done more searching before opening this. Is this still by design?

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.