DependentNull.java

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;
import org.checkerframework.checker.nullness.qual.*;
import org.checkerframework.framework.qual.Dependent;

public class DependentNull {

    /**
     * NOTE that @Prototype is a SUPERTYPE of an unannotated reference. (Uh, how does the checker
     * know that? It's important to the checking!)
     */
    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
    @interface Prototype {}

    private @NonNull @Dependent(result = Nullable.class, when = Prototype.class) String dep;
    @NonNull String indep;

    DependentNull() {
        dep = "hello";
        indep = "hello";
    }

    static void fieldAccess() {
        DependentNull t1 = new DependentNull();
        //:: error: (assignment.type.incompatible)
        t1.dep = null; // error
        t1.dep = "m";
        //:: error: (assignment.type.incompatible)
        t1.indep = null; // error
        t1.indep = "m";

        @Prototype DependentNull t2 = new DependentNull();
        t2.dep = null;
        t2.dep = "m";
        //:: error: (assignment.type.incompatible)
        t2.indep = null; // error
        t2.indep = "m";
    }

    void receiverNonProto() {
        //:: error: (assignment.type.incompatible)
        dep = null; // error
        dep = "m";

        //:: error: (assignment.type.incompatible)
        indep = null; // error
        indep = "m";
    }

    void receiverProto(@Prototype DependentNull this) {
        dep = null;
        dep = "m";

        //:: error: (assignment.type.incompatible)
        indep = null; // error
        indep = "m";
    }

    class Parameter {
        Parameter(@Dependent(result = Nullable.class, when = Prototype.class) String param) {}

        void use() {
            new @Prototype Parameter(null);
            //:: error: (argument.type.incompatible)
            new Parameter(null); // error

            new @Prototype Parameter("m");
            new Parameter("m");
        }
    }
}