create table T1 (
    ID   integer not null primary key,
    VAL  varchar(10)
);

create table T2 (
    ID   integer not null primary key,
    VAL  varchar(10)
);

set term ^ ;

create or alter procedure P
as
begin
  merge into T1 as t
  using ( with CTE ( ID, VAL ) as ( select ID, VAL from T2 )
          select ID, VAL from CTE ) as s
  on ( t.ID = s.ID )
  when matched then
    update set VAL = s.VAL;
end ^

set term ; ^

grant select,update on T1 to procedure P;
grant select on T2 to procedure P;
grant select on CTE to procedure P;

This operation is not defined for system tables.
unsuccessful metadata update.
table/view CTE does not exist.