PolyNullness.java

import org.checkerframework.checker.nullness.qual.*;

interface PolyFunc {
    @PolyNull String method(@PolyNull String in);
}

interface NonNullFunc {
    @NonNull String method(@NonNull String in);
}

interface MixedFunc {
    @NonNull String method(@Nullable String in);
}

class Context {

    static @PolyNull String poly(@PolyNull String in) {
        return in;
    }

    static String nonPoly(String in) {
        return in;
    }

    void context() {
        PolyFunc f1 = Context::poly;
        //:: error: (methodref.param.invalid)
        PolyFunc f2 = Context::nonPoly;

        NonNullFunc f3 = Context::poly;
        //:: error: (methodref.return.invalid)
        MixedFunc f4 = Context::poly;
    }
}