I understand that Java is generally invariant since it does not want to deal with issues of co/contra-variance.

So in general,

List<Parent> a = new ArrayList<Child>();

will require a type-erase-like cast via (List) to make work.

However,

List<Parent> p = Collections.singletonList(<Child>);

works as is, without a cast, despite claiming to return a List<Child> via the documentation.

I experimented a bit and found this is a property of generic returns in general, and is not specific to Collections.singletonList.

There are a couple possibilities here:

• it’s inferring the return type based on what it is being assigned to
  • this doesn’t make much sense, since context-free compilation wouldn’t work so well
  • a good point to be made is something like Vector<String> s = new Vector<>(); works like this
• it actually just returns List, type-erased, so the List<T> is a misnomer
  • this makes sense to me, but it’s strange that this is an exception case for the compiler and not the first assignment I gave as an example

What exactly is going on here, and why is it okay in this scenario, upon a function return with generics, but not with assignment in general?

Taking this one step further, how does Java implement and special-case this?