make lookup operator honor refined return type of "get()"

[jvasileff]

In "6.8.7. Correspondence, subrange, and stream operators", the lookup operator is defined as yielding the result of lhs.get(index) with type Y? for lhs Correspondence<X,Y>. But the actual return type of lhs.get(index) may be a subtype of Y?.

It would be nice if the lookup operator honored the actual return type of get rather than assuming an optional type.

Usage example:

object multimap satisfies Correspondence<String, [Integer*]> {
    shared actual Boolean defines(String key) => nothing;
    shared actual [Integer*] get(String key) => [];
}
[Integer*] result1 = multimap[""]; // error Integer[]? is not assignable to Integer[]
[Integer*] result2 = multimap.get(""); // ok

[gdejohn]

👍

I've had this same thought.