MemberReferences.java

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

abstract class References {

    void context(References c) {

        // No error
        FuncA funcA1 = References::aMethod1;
        // No error, covariant parameters
        FuncA funcA2 = References::aMethod2;
        //:: error: (methodref.return.invalid)
        FuncA funcA3 = References::aMethod3;
        //:: error: (methodref.return.invalid)
        FuncA funcA4 = References::aMethod4;

        //:: error: (methodref.param.invalid)
        FuncB funcB1 = References::aMethod1;
        // No error
        FuncB funcB2 = References::aMethod2;
        //:: error: (methodref.return.invalid) :: error: (methodref.param.invalid)
        FuncB funcB3 = References::aMethod3;
        //:: error: (methodref.return.invalid)
        FuncB funcB4 = References::aMethod4;

        FuncA typeArg1 = References::<@NonNull String>aMethod5;
        //:: error: (methodref.param.invalid)
        FuncB typeArg2 = References::<@NonNull String>aMethod5;
    }

    abstract @NonNull String aMethod1(@NonNull String s);

    abstract @NonNull String aMethod2(@Nullable String s);

    abstract @Nullable String aMethod3(@NonNull String s);

    abstract @Nullable String aMethod4(@Nullable String s);

    abstract <T> T aMethod5(T t);

    interface FuncA {
        @NonNull String method(References a, @NonNull String b);
    }

    interface FuncB {
        @NonNull String method(References a, @Nullable String b);
    }
}