Impossible to drop primary key in certain circumstances

create table test_a (id int primary key);

create table test_b (id int primary key);

create table test_a_b (a int not null,

                       b int not null,

                       constraint ref_a foreign key (a) references test_a (id),

                       constraint ref_b foreign key (b) references test_b (id));

alter table test_a_b add constraint a_b_pk primary key (a, b);

alter table test_a_b drop primary key;