another related bug, in master.
drop table if exists notnull_tbl1;
CREATE TABLE notnull_tbl1 (c0 int not null, c1 int);
ALTER TABLE notnull_tbl1 ADD CONSTRAINT Q PRIMARY KEY(c0, c1);
\d+ notnull_tbl1
ALTER TABLE notnull_tbl1 ALTER c0 DROP NOT NULL;
ALTER TABLE notnull_tbl1 ALTER c1 DROP NOT NULL;
"ALTER TABLE notnull_tbl1 ALTER c0 DROP NOT NULL;"
should fail?
I didn't investigate deep enough.