[Oberon] Serious type loophole in type case statements and a possible fix

[Andreas Pirklbauer]

    > I remember reading about a similar issue related to the WITH statement of
    > Oberon-90, so the way I solved this problem in oberonc is to add an additional
    > type test each time the case variable is referred in the statements of a particular case label.

Hi Luca,

Yes, that’s of course the obvious solution. But that would re-introduce run-time
overhead that the type case statement avoided in the first place. In that case, one
might as well get rid of the CASE altogether and just use IF p IS T THEN instead:

i.e. replace...

  PROCEDURE check0();
    BEGIN p0 := p1;
    CASE p0 OF
      P1:
        AssignGlobal();
        p0*(P1)*.fld1 := 123; 
    END
  END check0;

with...

  PROCEDURE check0();
    BEGIN p0 := p1; 
    IF p0 IS P1 THEN
        AssignGlobal();
        p0*(P1)*.fld1 := 123; 
    END
  END check0;

My (proposed) solution rescues the CASE statement (which is efficient relative
to IF x IS T + series of additional type tests inside the IF statement), while at
the same time solving its type loophole issues.

But I would not mind eliminating the type case statement and just have type guards.

-ap