First, you may already know, but for others: Using raw types is bad. We shouldn’t. Meaning we don’t need to know the answer to your question.

Curiosity doesn’t quite kill this cat, though, so for its sake: Once you use a raw type, everything in the expression goes raw. So when you pass a raw HashMap to doSometing(), it returns a raw List, not the List<B> that it’s declared to return. And Java accepts that you assign a raw List to a List<A>. With a warning. But you’re expected to know best, if you say that the list can be regarded as a List<A>, so be it.

In you last code line you haven’t got any raw types, so generics work the way they are supposed to. doSomething() returns a List<B>. And you cannot assign a List<B> to a List<A>. Why? Because a List<A> would allow you to insert something into the list that implements A but is not a B. We cannot allow that to happen for a List<B>.

Link: What is PECS (Producer Extends Consumer Super)? about why you can’t assign a List<B> to a List<A> and what to do instead.