The compiler ignores NOCOPY in these cases - oracle

Question

I read about IN OUT and NOCOPY. Then I encountered NOCOPY use cases but I was not able to get it. Can anybody explain these with examples? Thanks in advance.
The actual parameter must be implicitly converted to the data type of the formal parameter.
The actual parameter is the element of a collection.
The actual parameter is a scalar variable with the NOT NULL constraint.
The actual parameter is a scalar numeric variable with a range, size, scale, or precision constraint.
The actual and formal parameters are records, one or both was declared with %ROWTYPE or %TYPE, and constraints on corresponding fields differ.
The actual and formal parameters are records, the actual parameter was declared (implicitly) as the index of a cursor FOR LOOP statement, and constraints on corresponding fields differ.
The subprogram is invoked through a database link or as an external subprogram.

Answer 1

The basic principle is that PL/SQL will honour the NOCOPY directive as long as the value we pass can be used as provided, without transformation, and is addressable by the called program. The scenarios you list are circumstances where this is not the cases. I must admit a couple of these examples made me think, so this is a worthwhile exercise.
The first four examples call this toy procedure.
create or replace procedure tst2 (p1 in out nocopy t34%rowtype) is
begin
p1.id := 42;
end;
/
Case 1: The actual parameter must be implicitly converted to the data type of the formal parameter.
declare
n varchar2(3) := '23';
begin
tst(n);
dbms_output.put_line(n);
end;
/
Case 2: The actual parameter is the element of a collection.
declare
nt sys.odcinumberlist := sys.odcinumberlist(17,23,69);
begin
tst(nt(2));
dbms_output.put_line(to_char(nt(2)));
end;
/
Case 3: The actual parameter is a scalar variable with the NOT NULL constraint.
declare
n number not null := 23;
begin
tst(n);
dbms_output.put_line(to_char(n));
end;
/
Case 4: The actual parameter is a scalar numeric variable with a range, size, scale, or precision constraint.
declare
n number(5,2) := 23;
begin
tst(n);
dbms_output.put_line(to_char(n));
end;
/
The next example uses this table ...
create table t34 (id number not null, col1 date not null)
/
...and toy procedure:
create or replace procedure tst2 (p1 in out nocopy t34%rowtype) is
begin
p1.id := 42;
end;
/
Case 5 : The actual and formal parameters are records, one or both was declared with %ROWTYPE or %TYPE, and constraints on corresponding fields differ.
declare
type r34 is record (id number, dt date);
r r34;
begin
r.id := 23;
r.dt := to_date(null); --trunc(sysdate);
tst2(r);
dbms_output.put_line(to_char(r.id));
end;
/
The next example uses this package spec...
create or replace package pkg is
type r34 is record (id number, dt date);
end;
/
...and toy procedure:
create or replace procedure tst3 (p1 in out nocopy pkg.r34) is
begin
p1.id := p1.id + 10;
end;
/
Case 6: The actual and formal parameters are records, the actual parameter was declared (implicitly) as the index of a cursor FOR LOOP statement, and constraints on corresponding fields differ.
begin
for j in ( select * from t34) loop
tst3(j);
dbms_output.put_line(to_char(j.id));
end loop;
end;
/
The last example uses a remote version of the first procedure.
Case 7: The subprogram is invoked through a database link or as an external subprogram.
declare
n number := 23;
begin
tst#remote_db(n);
dbms_output.put_line(to_char(n));
end;
/
There are working demos of the first six cases on db<>fiddle here.