Built In Types: This

The type name this refers to the current class type at run time. As such, it can only be used from within a class, an interface, or a trait. (The type name this should not be confused with $this, which refers to the current instance, whose type is this.) For example:

interface I1 {
  abstract const type T1 as arraykey;
  public function get_ID(): this::T1;
}

Here, the function get_ID returns a value whose type is based on the type of the class that implements this interface type.

Strictly speaking, this is not a new type name, just an alias for an existing one.