Add Persistent::Upcast.

Add Value::Cast.

It's needed for upcasting Persistent<Object> to Persistent<Value> after
handlepocalypse (with Persistent::As or Persistent::Cast).

BUG=
R=svenpanne@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=14995

[marja, 2013-06-04 15:58:19]

dcarney, svenpanne, ptal.

[Sven Panne, 2013-06-05 07:08:49]

https://codereview.chromium.org/16206014/diff/1/include/v8.h
File include/v8.h (right):

https://codereview.chromium.org/16206014/diff/1/include/v8.h#newcode598
include/v8.h:598: static Persistent<T>& Upcast(Persistent<S>& that)) { // NOLINT
How does this relate to Persistent::Cast? It looks like it's the same, but with
an additional TYPE_CHECK. Do we really need this plethora of casting mehods
(Cast/As/Upcast)? This is currently a totally incomprehensible mess... :-/

Apart from that a tiny formatting nit: Breaking the line before V8_INLINE makes
the formatting less surprising.

[marja, 2013-06-05 07:22:35]

On 2013/06/05 07:08:49, Sven Panne wrote:
> https://codereview.chromium.org/16206014/diff/1/include/v8.h
> File include/v8.h (right):
> 
> https://codereview.chromium.org/16206014/diff/1/include/v8.h#newcode598
> include/v8.h:598: static Persistent<T>& Upcast(Persistent<S>& that)) { //
NOLINT
> How does this relate to Persistent::Cast? It looks like it's the same, but
with
> an additional TYPE_CHECK. Do we really need this plethora of casting mehods
> (Cast/As/Upcast)? This is currently a totally incomprehensible mess... :-/

Like the description says: "Value::Cast doesn't exist so we cannot use As or
Cast."

Both As and Cast end up trying to call Value::Cast, whereas Upcast doesn't.
Instead it checks that the types are fine, and then just reinterpret_casts.

[Sven Panne, 2013-06-05 07:30:29]

On 2013/06/05 07:22:35, marja wrote:
> On 2013/06/05 07:08:49, Sven Panne wrote:
> > https://codereview.chromium.org/16206014/diff/1/include/v8.h
> > File include/v8.h (right):
> > 
> > https://codereview.chromium.org/16206014/diff/1/include/v8.h#newcode598
> > include/v8.h:598: static Persistent<T>& Upcast(Persistent<S>& that)) { //
> NOLINT
> > How does this relate to Persistent::Cast? It looks like it's the same, but
> with
> > an additional TYPE_CHECK. Do we really need this plethora of casting mehods
> > (Cast/As/Upcast)? This is currently a totally incomprehensible mess... :-/
> 
> Like the description says: "Value::Cast doesn't exist so we cannot use As or
> Cast."
> 
> Both As and Cast end up trying to call Value::Cast, whereas Upcast doesn't.
> Instead it checks that the types are fine, and then just reinterpret_casts.

Why don't we add Value::Cast? (I haven't thought deeply about this.) As it is, I
don't want to obfuscate Persistent even further, 2 casting functions are not
good, 3 different casting functions are very bad. Please remember that we are
talking about a public API here, so we should be careful what we do here, people
might actually start using it...

[marja, 2013-06-06 14:49:54]

Alright, I don't know either why there was no Value::Cast. So I rewrote this CL
to add it, and not add Persistent::Upcast.

svenpanne, could you take another look?

[Sven Panne, 2013-06-07 06:56:02]

lgtm

[dcarney, 2013-06-07 08:46:45]

Committed patchset #3 manually as r14995 (presubmit successful).