[TCLCORE] Empty list is not a list -- using twoPtrValue for NULL type

[Alexandre F. <ale...@gm...>]

Hello,

While implementing mutability, I realize that the empty list at some
places (notably when doing Tcl_SetListObj with no elements) is not a
list at all, it's just an object with an empty string rep and a NULL
type.

That NULL type is nasty for me, because so far I relied on the intrep
to store the mutable flag (I don't want to increase the size of
Tcl_Objs).

Now, I could handle that case differently if I could find an unused
field in Tcl_Obj's of NULL type (pure strings). Is there one ? Is the
internalRep.twoPtrValue.ptr(1 or 2) usable for this purpose to denote
the fact that a NULL-type object is mutable ?

(BTW it's primarily intended for mutable empty lists, but could be
generalized to mutable strings)

Thanks for answering that one -- which is beyond grep :-)

-Alex