[sv-ec] $cast a null expression

hi EC

Last meeting we talked about how $cast should behave when its
first (target) argument is a class handle variable, and its
second (source) argument is an expression whose value is null.
I've created Mantis 3293 to capture this.

Given the increased complexity introduced here by the multiple
inheritance proposals, I'd be glad to know what behaviour
people think is desirable before I put any effort into a
proposal. It seems to me that there are several situations:

- If the source expression's statically determined type is a
   descendant of, or equivalent to, the target type, then a
   simple copy would be OK so it seems reasonable for the
   $cast to succeed unconditionally, and be compiled as a
   simple copy operation - so it would succeed if the source
   is null, too. Current tools have $cast succeed if the
   source is null, in this case.

- If the source expression's statically determined type is
   on a different branch of the hierarchy from the target's,
   then in SV-2009 the cast is sure to fail; having it succeed
   if the source happens to be null sounds unsatisfactory to me.
   However, if we have multiple inheritance then things are not
   so simple; the cast is no longer sure to fail, but I would
   guess that it should still be an error if the source is null.
   Current tools disagree about this already.

- If the source expression's statically determined type is
   an ancestor of the target's, what should happen if the
   source is null? Current simulators all say "cast succeeds",
   but is this correct?

- If the source expression is the special value "null", should
   the rules be any different than if the source is some other
   expression that happens to give a null value?

Finally, can someone kindly remind me about this subtlety, which
I seem to remember was dealt with for 2009 but I can't find the
relevant Mantis. Given these declarations:
   class C; ...
   class D extends C; ...
   C c;
   D d;
   bit b;
what do we say about the data type of this expression?
   b ? c : d
Is d effectively upcast to C before being returned, so the
expression as a whole has type C?

Thanks

-- 
Jonathan Bromley
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