I am not sure that this is a bug but it did behave in a way that I did not expect which is why I filed a new issue. If this is not a bug it should probably be closed and continued in earlier issues like #17678

TypeScript Version: 2.8.1

Search Terms:
Mapped types, never, filter, conditional types, Exclude, Omit, Pick

Code

type Fruit = { name: string };
type Apple = Fruit & { cultivar: string };

type Fruits = {
  banana: Fruit;
  alice: Apple;
  gala: Apple;
};

type Apples = {
  [T in keyof Fruits]: Fruits[T] extends Apple ? Fruits[T] : never
};

Expected behavior:
I expected Apples to have type

{
  alice: Apple;
  gala: Apple;
}

Actual behavior:
Apples have the type

{
  banana: never;
  alice: Apple;
  gala: Apple;
}

Having an object with a never key is quite strange to me. How would that object ever be created?

Playground Link: http://www.typescriptlang.org/play/#src=%0D%0Atype%20Fruit%20%3D%20%7B%20name%3A%20string%20%7D%3B%0D%0Atype%20Apple%20%3D%20Fruit%20%26%20%7B%20cultivar%3A%20string%20%7D%3B%0D%0A%0D%0Atype%20Fruits%20%3D%20%7B%0D%0A%20%20banana%3A%20Fruit%3B%0D%0A%20%20alice%3A%20Apple%3B%0D%0A%20%20gala%3A%20Apple%3B%0D%0A%7D%3B%0D%0A%0D%0Atype%20Apples%20%3D%20%7B%0D%0A%20%20%5BT%20in%20keyof%20Fruits%5D%3A%20Fruits%5BT%5D%20extends%20Apple%20%3F%20Fruits%5BT%5D%20%3A%20never%0D%0A%7D%3B%0D%0A

Related Issues:
If this have worked as expected #17678 would be already be supported. That issue is marked as a duplicate and linked to #12424 and #17636 but I am not sure I see the direct relation there to be honest.