[core] generic field read through a raw type is compared to 0 instead of null #2886

Issue details

Reading a generic field through a raw instance type and null-checking it emits == 0 instead of == null, with a type inference failed warning.
It affects both a plain type-variable field (T) and an array of one (T[]).

TypeBoundFieldGetAssign.getResultType (TypeBoundFieldGetAssign.java:42): when the field's type cannot be resolved from the (raw) instance type, replaceClassGenerics returns null and the fallback returns the field type unchanged (the existing // TODO: check if this type is allowed in current scope, line 47).
That leaves an out-of-scope type variable owned by another class; type propagation rejects it (TypeUpdate.checkForUnknownTypeVars), so the compared 0 literal keeps its unknown type and TypeGen renders it as 0.
The bound is built for any field whose type contains a type variable (TypeInferenceVisitor.java:271, containsTypeVariable()), so T[] is affected too.

Sample

Compile with javac and run jadx on the .class files:

public class GenBug {
    public static class Box<T extends Number> {
        public T value;
        public T[] arr;
    }
    public static boolean isNull(Box box) {   // raw Box
        return box.value == null;
    }
    public static boolean arrNull(Box box) {
        return box.arr == null;
    }
}

Current output on master (8c28a85):

public static boolean isNull(Box box) {
    return box.value == 0;   // expected: == null
}
public static boolean arrNull(Box box) {
    return box.arr == 0;     // expected: == null
}

Jadx version

master 8c28a85 (current HEAD)