[Oberon] Dynamic type of record var parameter

[Andreas Pirklbauer]

    > My understanding is that you need to use a type guard to
    > assert that p can be assumed to have a dynamic type of p2:
    >
    > i.e. p := q; check(p(p2)^);
    >

This would indeed produce the correct output (“is p2” instead of “is p0”)
in the provided test program.

However, the correct dynamic type could also be passed by the compiler
instead of relying on the programmer to “know” what the dynamic type is.

To fix this in the compiler, one needs to change procedure ORG.VarParam

From:

    PROCEDURE VarParam*(VAR x: Item; ftype: ORB.Type);
       VAR xmd: INTEGER;
     BEGIN ..
       ..
       ELSIF ftype.form = ORB.Record THEN
         IF xmd = ORB.Par THEN Put2(Ldr, RH, SP, x.a+4+frame); incR ELSE loadTypTagAdr(x.type) END
       END
     END VarParam;

To:

    PROCEDURE VarParam*(VAR x: Item; ftype: ORB.Type);
       VAR xmd: INTEGER;
     BEGIN ..
       ..
       ELSIF ftype.form = ORB.Record THEN
         IF x.deref THEN Put2(Ldr, RH, 0, -8); incR    (*pass the dynamic type rather than the static type*)
         ELSIF xmd = ORB.Par THEN Put2(Ldr, RH, SP, x.a+4+frame); incR
         ELSE loadTypTagAdr(x.type) END
       END
     END VarParam;

and make a few other small modifications to set x.deref whenever a pointer has been dereferenced.

The full solution can be found here:

    http://github.com/andreaspirklbauer/Oberon-allow-dereferenced-pointer-as-var-parameter

Andreas