A getter returning a map's mutable element: wrongly @UnmodifiableView, and modifying the element does not reach the owner #100

Two findings from one fernflower shape, StructContext.instanceOf: it takes a class from its map through a getter and calls a lazily-loading method on it.

class Ctx {
    static class C {
        private Object pool;
        Object getPool() { if (pool == null) pool = new Object(); return pool; }   // modifying
    }
    private final Map<String, C> classes = new HashMap<>();
    C getClass(String n) { return classes.get(n); }
    boolean direct(String n)         { C c = classes.get(n); return c.getPool() != null; }
    boolean viaGetter(String n)      { C c = getClass(n); return c.getPool() != null; }
    boolean viaGetterChain(String n) { return getClass(n).getPool() != null; }
}

1. A getter handing out a mutable element is @UnmodifiableView

getClass (and list.get(0) the same way) got unmodifiableViewMethod = true. TypeModIndyAnalyzerImpl.unmodifiableValue falls back to the callee's own verdict, and Map.get's IMMUTABLE_METHOD is immutable-HC: its result is a type parameter, the map's hidden content. Here the concrete element type C is mutable, so the result is no view. The IDE shows the wrong annotation.

Fixed on ws/maddi-bug: when the callee's formal result is a type parameter, the concrete type decides (at least immutable-HC: a view; otherwise not). Gate NOVIEWOFTYPEPARAMETER. Test TestUnmodifiableView.element (red with the gate).

2. Modifying an element handed out by a getter does not reach the map's owner (open)

The element is in this.classes' hidden content, so modifying it modifies this, deep-only, as direct shows. Through the getter the modification is lost: unsound. It is the element (∈) counterpart of #80 and #98, which handled a getter returning a field (≺). Not affected by fix 1.

How it was found

The rust-llvm oracle (maddi#167) on fernflower: StructContext::instance_of(&self) borrowed cl.get_pool() mutably through a shared borrow (E0596).

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