non-polymorphic Interface.outer ultimately causes NPE

[jvasileff]

This one is a definitely a corner case...

In the example below, given multiple candidate instances of OuterInterface to use as InnerInterface's outer, the wrong one is chosen. The most refined class should be used to as the starting point to determine the outer instance for an interface (working outward), whereas it appears the topmost satisfying class is used as the starting point (also working outward).

An instance of OuterClass should serve as InnerInterface.outer for instances of MiddleClass. This works.

But, an instance of MiddleClass should serve as InnerInterface.outer for instances of InnerClass. This does not seem to be working.

shared void run() {

    interface OuterInterface<out T> {

        shared default String outerInterfaceT => "OuterInterface.T = " + `T`.string;
        shared default String outerInterfaceIdent => "OuterInterface.ident, OuterInterface impl";

        shared formal T t;

        shared interface InnerInterface<out U> {

            shared default String innerInterfaceT => "InnerInterface.T = " + `T`.string;
            shared default String innerInterfaceU => "InnerInterface.U = " + `U`.string;

            shared default T outerT => outer.t;

            shared default String outerInterfaceTFromInnerInterface => outer.outerInterfaceT;
            shared default String outerIdentFromInnerInterface => outer.outerInterfaceIdent;
        }
    }

    class OuterClass() satisfies OuterInterface<Anything> {

        shared actual Anything t => null;
        shared actual String outerInterfaceIdent => "OuterInterface.ident; OuterClass impl";

        shared class MiddleClass() satisfies OuterInterface<Object> & OuterInterface<Anything>.InnerInterface<Object> {

            shared actual Object t => object {};
            shared actual String outerInterfaceIdent => "OuterInterface.ident; MiddleClass impl";

            // `t` should be an Anything from OuterClass's InnerInterface conformance
            shared default Anything someT => outerT;

            shared class InnerClass() extends MiddleClass() satisfies OuterInterface<Object>.InnerInterface<String> {
                // `t` should be an Object from MiddleClass's InnerInterface conformance,
                // not Anything from OuterClass's InnerInterface conformance
                shared actual Object someT => outerT;
            }
        }
    }

    // "Object" (correct), from MiddleClass satisfying OuterInterface
    print(OuterClass().MiddleClass().InnerClass().outerInterfaceT);

    // "Anything" (wrong), this is what we'd expect from MiddleClass's InnerInterface.outer,
    // but InnerClass's InnerInterface.outer should produce `Object`.
    print(OuterClass().MiddleClass().InnerClass().outerInterfaceTFromInnerInterface);

    // "...MiddleClass impl" (correct), from MiddleClass's override
    print(OuterClass().MiddleClass().InnerClass().outerInterfaceIdent);

    // "...OuterClass impl" (wrong), again what we'd expect from  MiddleClass's InnerInterface.outer
    print(OuterClass().MiddleClass().InnerClass().outerIdentFromInnerInterface);

    // "String" (correct), showing InnerClass actually does acknowledge the
    // refined satisfaction of InnerInterface, sometimes.
    print(OuterClass().MiddleClass().InnerClass().innerInterfaceU);

    // "Anything" (wrong), T==Anything is never paired with U==String
    print(OuterClass().MiddleClass().InnerClass().innerInterfaceT);

    // "<null>" (wrong, **Objects aren't Null!**)
    print(OuterClass().MiddleClass().InnerClass().someT of Object);


    // These are all normal and correct:
    print(OuterClass().MiddleClass().outerInterfaceT); // Object
    print(OuterClass().MiddleClass().outerInterfaceTFromInnerInterface); // Anything

    print(OuterClass().MiddleClass().outerInterfaceIdent); // MiddleClass impl
    print(OuterClass().MiddleClass().outerIdentFromInnerInterface); // OuterClass impl

    print(OuterClass().MiddleClass().innerInterfaceT); // Anything
    print(OuterClass().MiddleClass().innerInterfaceU); // Object

    print(OuterClass().MiddleClass().someT of Anything); // Null
}