select
    NULL IS DISTINCT FROM NULL,
    NULL IS NOT DISTINCT FROM NULL,
    Opaque(NULL) IS DISTINCT FROM NULL,
    Opaque(NULL) IS NOT DISTINCT FROM NULL,
    Opaque(NULL) IS DISTINCT FROM Opaque(NULL),
    Opaque(NULL) IS NOT DISTINCT FROM Opaque(NULL)
;

select
    NULL IS NULL,
    NULL IS NOT NULL,
    Opaque(NULL) IS NULL,
    Opaque(NULL) IS NOT NULL
;